user31571: In "Mastering Bitcoin", the author notes (source):

The maximum number of coins mined is the upper limit of possible mining rewards for bitcoin. In practice, a miner may intentionally mine a block taking less than the full reward. Such blocks have already been mined and more may be mined in the future resulting in a lower total issuance of the currency.

I understand that a possible answer is "just because" but I'm wondering if there are other reasons behind such an action.

user31570: Likely you're correct, it's not useful in practice. People could, however, use value destruction as an costly signal. 
Voting example; I run a poll of mining pools; number of Satoshis less than the full block reward, up to 5, is the vote for which of 5 proposed algorithms to run in the future. Once a majority of blocks over some time period vote for option X, everyone agrees to switch protocols. (This could allow identityless voting by miners.)
Commitment example; A miner can "promise" something, and commit the value destroyed as a signal that they are willing to do it. If a miner wants to extort the network, such commitment could be effective in showing that they are serious.

31571: <p>In "Mastering Bitcoin", the author notes (<a href="http://www.oreilly.com/catalog/errata.csp?isbn=0636920032281">source</a>):</p>

<blockquote>
  <p>The maximum number of coins mined is the upper limit of possible mining rewards for bitcoin. In practice, a miner may intentionally mine a block taking less than the full reward. Such blocks have already been mined and more may be mined in the future resulting in a lower total issuance of the currency.</p>
</blockquote>

<p>I understand that a possible answer is "just because" but I'm wondering if there are other reasons behind such an action.</p>
 31570: <p>Likely you're correct, it's not useful in practice. People could, however, use value destruction as an costly signal. </p>

<p>Voting example; I run a poll of mining pools; number of Satoshis less than the full block reward, up to 5, is the vote for which of 5 proposed algorithms to run in the future. Once a majority of blocks over some time period vote for option X, everyone agrees to switch protocols. (This could allow identityless voting by miners.)</p>

<p>Commitment example; A miner can "promise" something, and commit the value destroyed as a signal that they are willing to do it. If a miner wants to extort the network, such commitment could be effective in showing that they are serious.</p>
