Paperback Writer (or also known as SF Writer S.L Viehl) corrects herself here about the advice that you sell a book to a publisher first and then get an agent. An agent popped in to negate that, and I added that I’d also acquired my agent who then sold the book to a publisher, but I was a little out of it, because I didn’t phrase it too well.
But here are some reasons I think it might be good to hunt for the agent first, and then if you haven’t snagged yourself one, start harrying the publishers:
1) The agent will probably know the field better than you. The agent should know which editors are looking for/interested in/buying your type of novel, if not have some good leads on it, or an editor who’s very receptive to the agent because of good books the agent sent him/her in the past.
2) Your agent can simultaneously submit your novel to a number of editors. You can’t. That cuts down on the time spent marketing the book!
3) Your agent is more likely to understand the contract ins and outs
4) You are going to need one eventually, why not start now?
5) Many agents work very closely with their writers to bring the fiction up to the next level before submitting to the publisher (this could be good or bad, in my case and a couple of friends it was good)
6) Your agent should be more experienced at pitching books than you.
So there are 6 reasons I think having an agent first can sometimes be helpful.