Agents pitch your proposal or manuscript to publishers, try to get you the best deal, and negotiate your contract. They manage your business affairs with a publisher once the book is sold. Some agents actually edit their clients' book proposals, and give other substantive assistance as the proposal is being crafted. The answer to that question could take pages. Because agents have had long relationships with particular publishers and editors, an agent can sell a proposal faster than a writer can.
Proposals submitted by agents go to the top of the pile.
Agents' familiarity with publishing companies enable them to target book proposals to the most appropriate companies. A literary agent's negotiating skills can, hopefully but not always, get the writer at least a 15 percent larger advance, thus making the agent's commission immediately worthwhile. An agent does the tough negotiating with editors, so that the writer's relationship can be completely cordial. Agents know what clauses in a publisher's contract should be changed before the author signs. As more and more publishers become part of multi-national corporations, publishing contracts are becoming vastly complicated.