Hands have been incredibly difficult for me to learn to paint because I have never just simply hired a model. I like being alone while I work, so I spend a lot of wasted time contorting myself in front of a mirror and driving myself crazy. Lately things are easier because I've gotten a digital camera, so I can pose myself, or anyone else who will let me, the way I want them and then run to the computer and print myself a nice quiet model.
I guess hands are prominent because they're of course expressive, but also usefully counter-directive. Think of Leonardo's Annunciation. The Angel has landed, lily in hand; Mary knows what is going to happen but she plays it cool. Her face is blank enough for poker, but the war between her doubt and longing is revealed perfectly through her hands. Hands are useful in the same way that I sometimes use the third eye — an indicator of ambiguity. Of course I never think of all this stuff while I'm painting. I'm just putting two and two together here at the kitchen table while I'm writing.