paint drywall -- corners first?
It matters because a brush transfers more paint than a roller and leaves marks in the finish. By cutting in the edges first and following with the roller you spread the brushed on paint further leveling the finish while removing brush marks. In other words the rolled finish, which is what you want, goes as close to the corner as you can get without bumping. (about 1/4" away) If you roll first the corners would have a brushed finish the width of the brush, which is much wider.

The same argument applies, and is more obvious, in cases like textured finish. You want the final finish technique applied last, for maximum coverage.