Of the 600,000 new titles written each year, only about 60,000 get published. Your agent wants to make sure your among the 300-600 newly published authors. You want an agent who gets the most while giving away the least. Publishing contracts are legalistic nightmares. Your agent will obtain the best offers for you work, including subsidiary and derivative rights, and negotiate the best contract terms on you behalf.
Agencies operate differently and vary in their level of services to clients.
Before signing a contract make sure you learn the history of the agency, names of clients and titles of books recently sold. Ask for references and call clients to find out if the agent provides the level of service you are seeking. If it's a large agency with many agents, make sure you understand who will be representing your work. If you're not sure about the contract, seek help from a literary attorney. Worse than losing money, he had lost control of his manuscript.