user254: Another naive question...
Further to e.g. this answer, which may be an overview of what Mahayana teaches about emptiness, my question is why does Mahayana teach that?
I gather that the purpose of the anatta doctrine in the Pali canon is, at least partly if not wholly, to end suffering etc. -- e.g. MN 22:

It would make sense to grasp at a doctrine of self that didn’t give rise to sorrow, lamentation, pain, sadness, and distress.
Taṃ, bhikkhave, attavādupādānaṃ upādiyetha, yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā.
But do you see any such doctrine of self?
Passatha no tumhe, bhikkhave, taṃ attavādupādānaṃ yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā”ti?
No, sir.
No hetaṃ, bhante.
Good, mendicants!
Sādhu, bhikkhave.
I also can’t see any such doctrine of self.
Ahampi kho taṃ, bhikkhave, attavādupādānaṃ na samanupassāmi yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā.

What is the benefit if any of the emptiness doctrine? Does it have a "soteriological" purpose?
How does that work, i.e. what is the mechanism by which the doctrine is beneficial?
Or is it meant to be simply true, an observation or description of fact, without benefit?

user2282: 
Further to e.g. this answer, which may be an overview of what Mahayana teaches about emptiness, my question is why does Mahayana teach that?

Well, emptiness is not a Mahayana's invention. They just further elaborated on it. It's been there all along in the Buddha's Teaching. There's a whole Division in the MiddleLength Discourses dedicated to the theme, it's called the Division on Voidness - SunnataVagga, composed of MN 121 thru 130. While it's mainly part of the Insight side of training, but it's also an important theme for the cultivation of Sila and Samadhi:

"Bhikkhus, for direct knowledge of lust, three things are to be developed. What three? Emptiness concentration, markless concentration, and wishless concentration." ~~ AN 3.183 ~~


user-1: Good question. 
Emptiness is supposed to be fact regardless of its benefit. It brings vast benefit to see the emptiness of phenomena, of course, but it is not an invention with the purpose of being beneficial, just an explanation of the nature of phenomena. As the Upanishads say, 'the voidness of one is the voidness of all'. This would be just how it is. 
The subtlety here would be the non-difference of emptiness and appearance but this issue may not be relevant to your question.   
The answer to the question of why the Mahayana teaches anything is always that it is 'what is the case'. Naturally, not everyone agrees it is.   
The question about the soteriological benefit of realising emptiness is probably too difficult for a forum answer. A good commentary on Nagarjuna or Milarepa should cover it.  
EDIT: Comments from Chris rightly suggest that I should say more if this answer is to be useful. 
Emptiness is the absence of inherent existence. If suffering were not empty then there would be no path to its cessation. In relation to self and phenomena it would be precisely their emptiness that is the soteriological teaching of the Buddha. 
Here is Chandrakirti from his Entering the Middle Way: 

"Since selflessness is what liberates beings,
  The Buddha taught two types: the selflessness of individuals and of phenomena,
  Then, in order to better help those to be tamed,
  The teacher taught further classifications."

Even Nirvana would be empty. If the things that compose Samsara do not exist then how could there be a transcendence of them? In the Prajnaparamita Sutras the Buddha taught,

"Nirvana, too, is just like a dream." 

A Vaira song of realisation of the Lord Milarepa begins...

"From the standpoint of the truth that's genuine,
  There are no ghosts, there are not even buddhas,
  No meditator and no meditated,
  No paths and levels traveled and no signs,
  And no fruition bodies and no wisdoms,
  And therefore there is no nirvana there,
  Just designations using names and statements." 

The final verse reads...

"All of this a union vast and spacious,
  And all those skilled in realising this
  Do not see consciousness, they see pure wisdom,
  Do not see sentient beings, they see buddhas,
  Don't see phenomenon, they see their essence,
  And out of this compassion just emerges,
  Retention, powers, fearlessness and all
  The qualities embodied by a Buddha
  Just come as if you had a wishing jewel-
  This is what I, the yogi, have realised."        

Thus it would be precisely the emptiness of self and phenomena that is the soteriological message of the Middle Way doctrine.  
It may be noted that this view of phenomena allows to us to makes sense of change and motion which, as Western philosophers know to their cost, is paradoxical and incomprehensible for all other metaphysical views and doctrines. 
The essential idea required to make sense of emptiness intellectually in the the absence of any acquaintance, it seems to me, is that the genuine nature of Reality would be beyond all conceptual fabrication, unthinkable, unsayable, a unity free of all division and distinction. Such a 'phenomenon' could only be a conceptual void. Another way to say this would be to say that Reality is non-dual. Thus Nagarjuna's metaphysical scheme is shared by all non-dual traditions from advaita Vedanta to the Christianity of A Course in Miracles.           
These references come from The Sun of Wisdom: Teachings on the Noble Nagarjuna's Fundamental Wisdom of the Middle Way by Khenpo Tsultrim Gyamptso. I cannot recommend it highly enough.   
'  . 

user43: Short answer: because understanding that phenomena are not "solid", but are in fact an interpretation/imputation, removes craving and passions in regards to them, which prevents dukkha that arises from that.
Long answer:
Normally we think that world is like a fish tank - a 3D environment full of objects, some of which are food, some friends, some dangerous sharks etc. with ourselves being one of the fishes in the pool. Then we spend most of our lives chasing the food and attractive fish, avoiding sharks, and trying to occupy the best spot in aquarium.
We assume that the world actually exists as it appears to us. We take it for granted that objects are objects, that some of these objects are awesome and some are not, that we are in competition with the others, and that the objective of the game is to achieve happiness by arranging a perfect environment for ourselves, and then maintaining it as long as possible.
This entire dream is based on an idea of solidity of the external world, an idea that things are actually out there as they appear, awesome or bad independently of our mind, and that therefore obtaining them or avoiding them is the right thing to do, and that therefore failing at that is the right reason to be upset and to experience suffering.
However, attaining an insight into "how things really are" - which is to say in how things are not solid, and how the mind of illusory solidity develops (process known as D.O.) -- having a direct insight into all this pulls the rug from under the initial illusion of solidity and the entire complex of behavior that comes with it.
The traditional analogy is rope that seems like a snake. Once you see it's a rope, the fear of getting bitten is gone and has no basis for ever coming back.
The next question naturally is, what exactly are we talking about when we say things are not solid, and how does that help remove the craving and other kleshas.
When we say things are not solid, we are saying that each individual thing is in fact multiple independent phenomena that seem to be lumped together, but only when observed from a certain perspective. In fact, this perspective is the main force that gives things their illusory solidity. Or, put in other words, much of what things seem to be actually comes from ourselves, from our own interpretation. Remember the snake-rope.
Once we get a very clear first-hand insight into how we "project" (impute) our perspective onto things, and learn to loosen that up, we realize that much of the importance we assigned to things is in fact our own game of make-believe. We also realize how many conflicts and passions in the world originate from people taking their own perspectives/interpretations all too seriously and assuming things are the way they look to them, and that all other ways of seeing are wrong.
Just like individual phenomena, entire world as it appears in our subjective experience is a giant interpretation that we assemble by looking at things a certain way, and pulling together certain strings of ideas to form a (semi-)coherent picture. And then of course our idea of ourselves, our feeling of "me-ness", is similar construct, made up of a million separate strings we strive to maintain intact. Emptiness of self is studied at length in Pali Canon, under the name of Anatta.
What Buddhists call the circle of Samsara (the cycle of subsequent generations of people inheriting this ignorant way of perception and passing it on to their children) and the struggle that comes with it - entirely depends on ignoring the fact that reality is our interpretation and that our interpretation is based on our limited exposure, and that our exposure comes from the circumstances, and that our circumstances come from results of previous actions, and that previous actions came from taking the illusion of solidity at the face value.
But once we understand how it all works, we can be free from it to a large degree, at least we can certainly be free from the subjective dukkha, and oftentimes even free from getting ourselves into troublesome situations that involve conflict of ideas.
Why? Because when we see that things are our own projection/interpretation/imputation - we no longer are seriously attached to them, we are more flexible, we allow for other interpretations, we see things from multiple perspectives which is a more robust way of seeing things. It turns out that most of our cravings, conflicts, and passions - most of our trouble and suffering - came from the assumption that things are real and actually possess all those qualities and characteristics, and now that we know they are/do not, now we can be free from all that.

254: <p>Another naive question...</p>

<p>Further to e.g. <a href="https://buddhism.stackexchange.com/a/26040/254">this answer</a>, which may be an overview of <strong>what</strong> Mahayana teaches about emptiness, my question is <strong>why</strong> does Mahayana teach that?</p>

<p>I gather that the <em>purpose</em> of the anatta doctrine in the Pali canon is, at least partly if not wholly, to end suffering etc. -- e.g. <a href="https://suttacentral.net/mn22/en/sujato" rel="nofollow noreferrer">MN 22</a>:</p>

<blockquote>
  <p>It would make sense to grasp at a doctrine of self that didn’t give rise to sorrow, lamentation, pain, sadness, and distress.<br>
  <em>Taṃ, bhikkhave, attavādupādānaṃ upādiyetha, yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā</em>.</p>
  
  <p>But do you see any such doctrine of self?<br>
  <em>Passatha no tumhe, bhikkhave, taṃ attavādupādānaṃ yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā”ti?</em></p>
  
  <p>No, sir.<br>
  <em>No hetaṃ, bhante.</em></p>
  
  <p>Good, mendicants!<br>
  <em>Sādhu, bhikkhave.</em></p>
  
  <p>I also can’t see any such doctrine of self.<br>
  <em>Ahampi kho taṃ, bhikkhave, attavādupādānaṃ na samanupassāmi yaṃsa attavādupādānaṃ upādiyato na uppajjeyyuṃ sokaparidevadukkhadomanassupāyāsā.</em></p>
</blockquote>

<p>What is the benefit if any of the emptiness doctrine? Does it have a "soteriological" purpose?</p>

<p>How does that work, i.e. what is the mechanism by which the doctrine is beneficial?</p>

<p>Or is it meant to be simply true, an observation or description of fact, without benefit?</p>
 2282: <blockquote>
  <p>Further to e.g. this answer, which may be an overview of what Mahayana teaches about emptiness, my question is why does Mahayana teach that?</p>
</blockquote>

<p>Well, emptiness is not a Mahayana's invention. They just further elaborated on it. It's been there all along in the Buddha's Teaching. There's a whole Division in the MiddleLength Discourses dedicated to the theme, it's called the Division on Voidness - <a href="https://suttacentral.net/mn-sunnatavagga" rel="nofollow noreferrer">SunnataVagga</a>, composed of MN 121 thru 130. While it's mainly part of the Insight side of training, but it's also an important theme for the cultivation of Sila and Samadhi:</p>

<blockquote>
  <p>"Bhikkhus, for direct knowledge of lust, three things are to be developed. What three? Emptiness concentration, markless concentration, and wishless concentration." ~~ <a href="https://suttacentral.net/an3.183-352/en/bodhi" rel="nofollow noreferrer">AN 3.183</a> ~~</p>
</blockquote>
 -1: <p>Good question. </p>

<p>Emptiness is supposed to be fact regardless of its benefit. It brings vast benefit to see the emptiness of phenomena, of course, but it is not an invention with the purpose of being beneficial, just an explanation of the nature of phenomena. As the Upanishads say, 'the voidness of one is the voidness of all'. This would be just how it is. </p>

<p>The subtlety here would be the non-difference of emptiness and appearance but this issue may not be relevant to your question.   </p>

<p>The answer to the question of why the Mahayana teaches anything is always that it is 'what is the case'. Naturally, not everyone agrees it is.   </p>

<p>The question about the soteriological benefit of realising emptiness is probably too difficult for a forum answer. A good commentary on Nagarjuna or Milarepa should cover it.  </p>

<p><strong>EDIT</strong>: Comments from Chris rightly suggest that I should say more if this answer is to be useful. </p>

<p>Emptiness is the absence of inherent existence. If suffering were not empty then there would be no path to its cessation. In relation to self and phenomena it would be precisely their emptiness that is the soteriological teaching of the Buddha. </p>

<p>Here is Chandrakirti from his <em>Entering the Middle Way</em>: </p>

<blockquote>
  <p>"Since selflessness is what liberates beings,<br>
  The Buddha taught two types: the selflessness of individuals and of phenomena,<br>
  Then, in order to better help those to be tamed,<br>
  The teacher taught further classifications."</p>
</blockquote>

<p>Even Nirvana would be empty. If the things that compose Samsara do not exist then how could there be a transcendence of them? In the <em>Prajnaparamita Sutras</em> the Buddha taught,</p>

<blockquote>
  <p>"Nirvana, too, is just like a dream." </p>
</blockquote>

<p>A Vaira song of realisation of the Lord Milarepa begins...</p>

<blockquote>
  <p>"From the standpoint of the truth that's genuine,<br>
  There are no ghosts, there are not even buddhas,<br>
  No meditator and no meditated,<br>
  No paths and levels traveled and no signs,<br>
  And no fruition bodies and no wisdoms,<br>
  And therefore there is no nirvana there,<br>
  Just designations using names and statements." </p>
</blockquote>

<p>The final verse reads...</p>

<blockquote>
  <p>"All of this a union vast and spacious,<br>
  And all those skilled in realising this<br>
  Do not see consciousness, they see pure wisdom,<br>
  Do not see sentient beings, they see buddhas,<br>
  Don't see phenomenon, they see their essence,<br>
  And out of this compassion just emerges,<br>
  Retention, powers, fearlessness and all<br>
  The qualities embodied by a Buddha<br>
  Just come as if you had a wishing jewel-<br>
  This is what I, the yogi, have realised."        </p>
</blockquote>

<p>Thus it would be precisely the emptiness of self and phenomena that is the soteriological message of the Middle Way doctrine.  </p>

<p>It may be noted that this view of phenomena allows to us to makes sense of change and motion which, as Western philosophers know to their cost, is paradoxical and incomprehensible for all other metaphysical views and doctrines. </p>

<p>The essential idea required to make sense of emptiness intellectually in the the absence of any acquaintance, it seems to me, is that the genuine nature of Reality would be beyond all conceptual fabrication, unthinkable, unsayable, a unity free of all division and distinction. Such a 'phenomenon' could only be a conceptual void. Another way to say this would be to say that Reality is non-dual. Thus Nagarjuna's metaphysical scheme is shared by all non-dual traditions from <em>advaita</em> Vedanta to the Christianity of <em>A Course in Miracles</em>.           </p>

<p>These references come from <em>The Sun of Wisdom: Teachings on the Noble Nagarjuna's Fundamental Wisdom of the Middle Way</em> by Khenpo Tsultrim Gyamptso. I cannot recommend it highly enough.   </p>

<p>'  . </p>
 43: <p>Short answer: because understanding that phenomena are not "solid", but are in fact an interpretation/imputation, removes craving and passions in regards to them, which prevents dukkha that arises from that.</p>

<p>Long answer:</p>

<p>Normally we think that world is like a fish tank - a 3D environment full of objects, some of which are food, some friends, some dangerous sharks etc. with ourselves being one of the fishes in the pool. Then we spend most of our lives chasing the food and attractive fish, avoiding sharks, and trying to occupy the best spot in aquarium.</p>

<p>We assume that the world actually exists as it appears to us. We take it for granted that objects are objects, that some of these objects are awesome and some are not, that we are in competition with the others, and that the objective of the game is to achieve happiness by arranging a perfect environment for ourselves, and then maintaining it as long as possible.</p>

<p>This entire dream is based on an idea of solidity of the external world, an idea that things are actually out there as they appear, awesome or bad independently of our mind, and that therefore obtaining them or avoiding them is the right thing to do, and that therefore failing at that is the right reason to be upset and to experience suffering.</p>

<p>However, attaining an insight into "how things really are" - which is to say in how things are not solid, and how the mind of illusory solidity develops (process known as D.O.) -- having a direct insight into all this pulls the rug from under the initial illusion of solidity and the entire complex of behavior that comes with it.</p>

<p>The traditional analogy is rope that seems like a snake. Once you see it's a rope, the fear of getting bitten is gone and has no basis for ever coming back.</p>

<p>The next question naturally is, what exactly are we talking about when we say things are not solid, and how does that help remove the craving and other kleshas.</p>

<p>When we say things are not solid, we are saying that each individual thing is in fact multiple independent phenomena that <em>seem</em> to be lumped together, but only when observed from a certain perspective. In fact, this perspective is the main force that gives things their illusory solidity. Or, put in other words, much of what things seem to be actually comes from ourselves, from our own interpretation. Remember the snake-rope.</p>

<p>Once we get a very clear first-hand insight into how we "project" (impute) our perspective onto things, and learn to loosen that up, we realize that much of the importance we assigned to things is in fact our own game of make-believe. We also realize how many conflicts and passions in the world originate from people taking their own perspectives/interpretations all too seriously and assuming things are the way they look to them, and that all other ways of seeing are wrong.</p>

<p>Just like individual phenomena, entire world as it appears in our subjective experience is a giant interpretation that we assemble by looking at things a certain way, and pulling together certain strings of ideas to form a (semi-)coherent picture. And then of course our idea of ourselves, our feeling of "me-ness", is similar construct, made up of a million separate strings we strive to maintain intact. Emptiness of self is studied at length in Pali Canon, under the name of Anatta.</p>

<p>What Buddhists call the circle of Samsara (the cycle of subsequent generations of people inheriting this ignorant way of perception and passing it on to their children) and the struggle that comes with it - entirely depends on ignoring the fact that reality is our interpretation and that our interpretation is based on our limited exposure, and that our exposure comes from the circumstances, and that our circumstances come from results of previous actions, and that previous actions came from taking the illusion of solidity at the face value.</p>

<p>But once we understand how it all works, we can be free from it to a large degree, at least we can certainly be free from the subjective dukkha, and oftentimes even free from getting ourselves into troublesome situations that involve conflict of ideas.</p>

<p>Why? Because when we see that things are our own projection/interpretation/imputation - we no longer are seriously attached to them, we are more flexible, we allow for other interpretations, we see things from multiple perspectives which is a more robust way of seeing things. It turns out that most of our cravings, conflicts, and passions - most of our trouble and suffering - came from the assumption that things are real and actually possess all those qualities and characteristics, and now that we know they are/do not, now we can be free from all that.</p>
