Watch the The Capo Crusader online guitar lesson by Rich Maloof from Guitar 201: Breakthroughs
Certain artists use capos all the time, which often means they're finding those step-ups more comfortable for their vocal range. Another very cool application, though, is using the capo to create a new part when you're playing with another guitarist. You can play the same chords as the other player but in an entirely different position, which can make for really nice, intertwined parts. It lends a broad sound to every chord to hear it voiced in multiple positions.