Because situational awareness. No matter what vehicle I'm driving, I want to know where every vehicle near me is and what they're doing from moment to moment. Without a mirror I'd have to be turning my head every 5 seconds or so.

While I'm turning my head, I'm not able to see much of anything (you think you can, but your brain is just blanking out and filling in the blur your eyes are sending it). If I use a mirror I can check behind me in a fraction of a second, MUCH MUCH faster than turning my head and without taking my eyes completely off the road in front of me or to my right.