Literary agents are the book world’s talent scouts. Most editors are too busy to read books sent in by unpublished authors, so only read submissions given to them by agents. That way, they know there’s already been some quality control.
Agents don’t just help you get a book deal. They will give you editorial help, check contracts, negotiate translation and film rights, chase up payments and sometimes get into arguments with your publisher if there’s a problem. Because that’s another thing: getting published is only the first step. Things go wrong. Projects get cancelled, sales figures might not be as good as everyone hoped. You might get given a cover you don’t like, or a bad review. A good agent will support you through the tough times as well as the good ones. A writer usually has several editors over the course of his or her career, but if they’re lucky, they stay with one agent.