This is a common question, and very apropos given the terrible budget news we got about NASA earlier today. So I put my answer up on YouTube. We'll be hearing this argument a lot in the upcoming budget battles, and hopefully this'll help show why we need to spend money on NASA, even more than we are now.
I have a lot of blog posts dealing with this issue, since it comes up so often. You'll find all them linked in this blog post: Debating Space. We need to be exploring space, and we need to be investing in our future. Science and exploration are our future.