Established literary agents have connections to acquisitions editors at large publishing houses, who may not otherwise accept submissions. They can pitch your book to the people who are most likely to be interested.
Because most writers are not lawyers, they may find themselves needing outside assistance when it is time to negotiate a contract. A good literary agent will work to get the best terms possible for their client, including an increased advance or retention of foreign rights. Agents know what clauses are important to strike from contracts.
Literary agents are in the business of selling books and they understand how to make your work more appealing to publishers. Though agents are not editors, they may offer suggestions to clients and potential clients about revisions that would improve their product.
Agents will receive checks from your publishing house and send you the money. If there is a discrepancy or error on your royalty statement, your literary agent will approach the house so you don't have to.
Most writers do not have the time, connections or expertise to sell foreign rights in the same way a literary agent can. Selling rights within other countries can be a lucrative way to increase the earnings from your book.