Just as in real life, characters in fiction need jobs. They need something to do besides sit around, eat bonbons and worry about their troubles. Unless they're independently wealthy (which happens sometimes) or landed gentry (which also happens sometimes), they need a visible means of supporting themselves. Even if what they do is not quite on the up-and-up, they still have to pay bills and eat.
How they spend their time, even off camera (outside of the story), goes to what's at the heart of all fiction: a character's desire. What he wants, and what he's willing to do to get it. Jobs of all types can add nuance and complication. If the hero is a bartender, what's it like for him? Does he wish he were doing something else? If so, what? Does he have hobbies competing for his time and attention?
Jobs also give characters a reason to be or not be at certain places at certain times. And in some books, a character's job may take center stage, molding the character's conflicts and tensions.
When I begin fleshing out a story, I have a character interview sheet I use that has all those questions about things I should know about my character, from What color are his eyes to What's his favorite color/food/movie, etc. All of these sorts of questions help shape the character. But to really find out who a character is, I pay close attention to what he does, and why he does it. Let's face it. A character who has to be at the hospital or library or research lab has a life--one the reader can be drawn into. Occupation adds dimension.
Occupation gives purpose. The hero can't just lay around butt-naked all day. Well, he can...
... but he really needs to be more than just a pretty face attached to a muscular body. Really. He does.
I'm over at the NCP blog today, talking about men. What else?!? Come on over!
"Love is... born with the pleasure of looking at each other, it is fed with the necessity of seeing each other, it is concluded with the impossibility of separation." ~Jose Marti