Q:

What was the motivation to have the Plan B in Interstellar

Spoilers coming, please keep that in mind if you haven't see the movie yet.
So the Plan B in Interstellar is to save mankind from extinction by making a new population from embryos. What is the point of that? All the people on Earth will die, (except for the staff of the Endurance), and babies born on the alien habitable planet never actually lived in mankind. So in fact, mankind, as we know, will become extinct and a new mankind will rise. 
Considering the fact that we polluted our planet and the space around it caused many species to go extinct and do a big damage to nature. Why would anyone be motivated to save this self-destructing mankind?

A:

Well, you're adressing the same problem the humans in this movie are aware of, too. As you said, all the people on earth would die and there would simply be a new mankind. "That is why Plan A is much more fun" and why everyone involved is so desperate to get Plan A working and so devastated when they learn it's supposedly impossible. The whole point of Plan B is to look far inferior to Plan A.
However, it at least helps to preserve humanity as a species, even if not the individual persons you and I know. In this hopeless time it is just the last straw to preserve at least something from us and to not go extinct. It is not so much about individual survival, but survival of our significance to the universe. Mankind just tries to not vanish from this universe without a sign left, but to still take part in it. If all the humans on earth have to die, then we at least want our genome and part of our spirit to remain. Sure, it's only the last straw, surviving ourselves would definitely be better. But at some point

We must think not as individuals, but as a species.

It is actually interesting that you ask this, since it is also mentioned in the movie that there is a psychological barrier in us humans that doesn't let us care about the future of mankind if we're not directly involved and that we have yet to overcome. If we ourselves or our beloved ones don't have any hope for survival, we don't have much inclination to save those after us either. That is the reason why Plan B is so hard to do and why Dr. Brand and Dr. Mann kept it unknown from the team that Plan A won't work at all. Or as Dr. Mann puts it (though, he might have a little cynical attitude towards that, as he's himself by far not a prime example of mankind in this regard):

He knew how much harder it would be for people to come together to save the species, instead of themselves, or their children...Would you have left if you hadn't believed you were trying to save them? Evolution has yet to transcend that simple barrier -- we can care deeply, selflessly for people we know, but our empathy rarely extends beyond our line of sight.

And I think this closes the circle very well to you asking that exact same question, why care about Plan B and any new mankind at all? You're supposed to ask this. And maybe the only way to answer it would be to transcend into that understanding of humanity not just as you and me but more than that. But fortunately at the moment we don't have to do that (and the crew ultimately didn't either).

But as to your last point about polluting our planet and destroying nature. That is an angle that the movie clearly and deliberately does not explore at all. At no point does the movie ask the question if the whole situation is actually our fault and it doesn't want to ask this question, since it is a story about the progress of humanity to new directions and higher levels, not about looking back and asking what we did wrong. This is also reinforced by screenwriter Jonathan Nolan in an interview with Jordan Goldberg (quoted from the book Interstellar: The Complete Screenplay with Selected Storyboards):

We're sort of in this moment in which humans are obsessed that we'll prove our own undoing -- that we'll poison the planet, we'll destroy ourselves, and all these things. But I thought it would be more interesting to find a slightly less personal Armageddon, or the idea that the universe obliterates you or the planet turned itself toxic because it doesn't care about you and me because we're an accident in outer space. The blight and dust provided what I thought was a great, impersonal way for the planet to sort of gently suggest that our time here was over. That it was the moment to move on, rather than being something that we had brought on ourselves which, in its own way, feels anthropocentric.

