I have an agent in London since about 6 months back. They've been extremely helpful and supportive although they haven't gotten me any jobs yet. But that's not really their job in the end: their job is to put me in context, so to speak.
They represent about 20 different cinematographers. Some guys just do feature film work, others do only commercials and a few of us do promo's. A couple of these guys like Steve Chivers and David Ungaro and others are all real commercial pro's and they're always booked up.
The thing that's good about an agent is basically that much like a cinemaplex, where if your film is sold out, there are others readily available.
Say that client A wants Bigshot DP A for their commercial. He's in Bahamas shooting Nike commercials for 3 weeks so they can't use him. "Ok, then give us Bigshot DP B instead". Unfortunately, he's in Macchu Picchu with Spike Jonze shooting hummingbirds. "Ok, so what else you got?", the client says. That's when your agent mentions that they've got this brilliant South African promo DP who is very good. All of a sudden, the little swedish cinematographer Adam Frisch gets a shot at shooting a rather big promo that the "brilliant South African" DP had to turn down because he got his first really big commercial with the really hot production company. That's how it works many times - work trickles down.
Me, I have to do pavement pounding in London. I do all the grunt work myself, book meetings, go and show my reel to people and so on. My agent doesn't get me jobs, I have to do that. Which is fine for me, I never expected them to do that anyway. I know the film industry - it's as much about your personality as it is about how good you shoot. A personal contact is much more important than a great reel.
Be nice, be humble, meet people and let them know that you're a good guy and that you know how to shoot. Sooner or later that'll get you work.
So the role of an agent is to put you in context with other shooters. They'll also help you with contacts, like whom to call and whom to meet. They'll send your reel to companies, book you meetings and negotiate fee's for you. All this is very helpful and worthwile. And for me, being new in London, it's easily worth 10%.