Another reason is that the earth's crust is quite rare in virtually all precious and useful metals (just look at the d-block of the periodic table for examples). Virtually all of them sank to the core during earth's formation, the existing deposits are the result of asteroids striking. So, asteroid mining is worth considering even if you're a pure capitalist working for your own gain.
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.