At what stage should a new writer think about using an agent? If, hypothetically, a new writer has a paperback coming out in 2008 with a major publisher, and a YA novel contracted for 2009 with another major publisher, and several things about to be released by the Educational press, and two already published items in the US Chicken Soup for the Soul Series, should he/she have an agent? What if he/she currently has two other manuscripts being considered by said two publishers? Still speaking hypothetically, what can an agent give a beginner who has worked really hard on his/her own behalf so far, that he/she has not been able to achieve?
The first thing to deal with here is the term 'new writer', because it affects how the question is answered. The hypothetical author mentioned above isn't really a 'new' writer because s/he already has books heading for publication. But a really new writer - one who has had no contact or contract with publishers - is a different case, so I'll straight to the questions at the end: what can an agent give a beginner who has worked really hard on his/her own behalf so far, that he/she has not been able to achieve?
If the beginner is not the hypothetical author with the contracts - if s/he has been working hard on their own behalf but hasn't yet found a publisher - then an agent can offer them access to publishers, and to the right publisher (who is not necessarily the publisher with the biggest advance). About half of an agent's work is with publishers and half with authors. The half that is to do with publishers is often just staying in touch, finding out what they're up to and what they're looking for, so that when one of the authors has a manuscript to send out, it goes to the publishers most likely to consider it seriously. Each agent could send out a manuscript willy-nilly, but that's wasting everyone's time. The other aspects are the deal, the contract and the ongoing relationship with the publisher. An unagented author may not know what to ask for when they're made an offer by a publisher - which rights should they keep, for example? And they need to look carefully at the contract - sometimes I will look over contracts for authors who aren't clients but need another eye on the contract, and I'm always amazed by what's in those contracts. As to the relationship with the publisher - usually things go well, because publishers are well behaved in general, but when something goes wrong you may need a third party to help sort it out.
Which brings me to our hypothetical author with lots of contracts. This person is doing pretty well for themselves - they're obviously good at getting to the right people in a publishing company and at promoting their own work. That's fantastic - but atypical. Most authors, particularly when they are starting out, aren't sure how to do any of this. But if you're the author who does, then you may not need an agent. You'll probably only need an agent if you'd prefer to have someone else handling your contracts and making sure your subsidiary rights are being looked after, or - because there are multiple publishers - if you'd rather have someone else do all the talking for you. Ideally the agent frees up the author to concentrate on the creative work while we take care of the business (and also advise on the creative). If our hypothetical author likes the business side, though, and they're happy with their publisher/s, they don't really need an agent.