The thing with looking at rocks and stuff on our planet is that we have weather that constantly erodes them and we have a molten core that makes a rock stew that gets spewed out of volcanoes. Asteroids, on the other hand, are sailing around through empty space looking just the way they did when they were formed four and a half billion years ago. So we can learn a lot about the formation of the universe by looking at those rocks that have been frozen in time.