Using mirrors in the studio allows for a fresh look at the proportions and color combinations of your painting. I'm always shocked at the reflection, it looks so new every time.
I think this is a common issue for artists, staring at the same thing for hours and days makes it hard to see how you can change certain elements or even that they need changing at all. Using a mirror is one way to fix this problem quickly, although sometimes you really just need to step away from the painting for a few days so as to see it in a new light. Occasionally it seems like even days are not enough to gain a new perspective, for instance I've heard stories of Andrew Wyeth going into galleries with his paints and "fixing" some of his work because he saw a new flaw in it.