Earth is currently the only known biosphere. More biospheres means that disasters that muck up one are less likely to muck up everything.
Less seriously, people like things that are cool.
EDIT: Seriously? My most-upvoted comment of all time? Really? This is as good as it gets?
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.