It's not that different from a skilled typist still looking at his hands once in a while. Besides the force of habit and natural tendency to look at your hands while working, it sometimes helps improve accuracy. I can type blind, but I often look down when typing in a new password. I can also play blind, but when it's not a very well rehearsed riff, I feel that looking at my left hand improves my accuracy and helps me focus on what I'm doing.
(but of course, if you're playing a guitar by vision alone, you're no better than the hunt-and-peck typist)