When I first signed with an agent around 1990, my agent insisted on things I didn't have the courage to ask for or the knowledge to consider. I'll give you a few of them.
The most obvious is the royalty rate. Sometimes agents can negotiate that, but not always. They can, however, find other ways to benefit writers. For example, free copies of books. Most publishers grant 10 to 25 freebies, but an agent might ask for 200. (That's the number of free copies my agent asked for and received for my second book with Dr. Ben Carson, Think Big.) Not a lot of money, but I sold or gave away those books.
Think of the importance of subsidiary rights. For example, I sold Gifted Hands before I had an agent. The original publisher was Review & Herald. They published the book in hardback, and sold the subsidiary rights to Zondervan. The book has remained in print since 1990 in hardback, soft cover, and mass paper. HarperCollins picked up the mass paper edition and sold 90,000 copies.
The downside is that Ben Carson and I receive only 50 percent of the royalties from Zondervan and HarperCollins. Standard contracts give the original publisher 50 percent of the royalties paid by the subsidiary publisher. Despite that, Ben and I have done well, but we would have done better if we had known.
Some publishers won't negotiate on the subsidiary rule and most of our books don't get picked up by another publisher. Even so, a good agent can sometimes get that 50 percent knocked down so the writer receives 70 percent.
Another area involves movie and electronic rights as well as any other medium. When we sold Gifted Hands, Ben Carson insisted on retaining the movie rights. The publisher resisted but gave in and 19 years later, Johnson & Johnson sponsored a made-for-TV version with Cuba Gooding Jr.
Good literary agents know the parts of a contract worth negotiating.