I think the question can be turned around and just as fairly asked to those who oppose spaceflight in favor of some earthly utopia. To what end? Even if it were possible to create a steady state society (which I doubt) that eliminated human suffering what would we all do?

I think we see the answer in unhealthy pastimes like gambling and extreme sports and other reckless activity. The only way people find the best in themselves is to have to work as a team to solve challenges not even imagined when they started.

I'm not sure of the exact quote but Raymond Massey spoke the H.G. Wells words in "Things to Come" to the effect that we must go forward or die, it is the stars or nothing.

Although it is fair question in some senses, I think the ultimate answer beyond just material improvements is that we will be a better, more complete species, representatives of the only known examples of life. We will tell the unknowing and uncaring universe that we are here and there is NOTHING that we can't accomplish.