An agent is relatively new entity in the world of sports. They became popular in the 1970s and have increased in power and prestige over the years. The modern athlete won't make a move without getting his agent's approval. They hold sway over many decisions, sometimes causing professional teams to shift into making better trades.
An agent can take heat off his client for many reasons. Athletes are traditionally not strong businessmen; they spend too much time training and working out to hone their athletic ability. Thus, an agent can take care of the business decisions that most athletes are either ill-equipped to do or unwilling to do.
The athlete also enjoys playing the role of a beloved figure in the town. They don't want to be seen as the bad guy. So when it comes to asking for money or asking for a better trade, they prefer to have an agent as the front man. This means the agent seems like the bad guy, not the player, even though the athlete is usually the one who is making the requests. This way the athlete doesn't get booed on the field or on the court or ravaged in the press. The agent takes the bullet.
Agents are a great help when it comes to negotiating a contract. Many have been to law school and understand how to structure contractual agreements that make them both lucrative and easy to escape from, should the need for more money arise.
An agent can be an individual with a few clients or an agent can belong to a larger firm that handles many athletes. Some agents specialize by sport, preferring to make better trades on behalf of their clients in the field of football or baseball or golf. There are even agents and agencies who only represent coaches, a growing area of clientele that often needs protection.
In addition to dealing with contracts, an agent will often help their client with investment opportunities, taxes and endorsements. Their job is to make the player took good, make money and have plenty of cash when they get ready to retire.
Several books have been written by or about agents and how their better trades practices work. Among the best are "Career Spotlight: David Falk" by Dan Migala. A popular movie about an agent and his relationship was the 1996 Jerry Maguire, which starred Tom Cruise and won an Academy Award for Cuba Gooding Jr. The movie made the catch phrase, "Show me the money," a part of the American vernacular.