If IT contractors don't understand agencies, and how to use them best, it will be difficult to have a successful contracting career. So, don't think of them as being like actors' agents or footballers agents. Naive young contractors believe that they are their agents like footballers agents and will look after their interests. Those agents want to get their clients the best deals they can. Recruitment agents want to get contractors work but they also want to take as big a slice of the rate cake as they can.
It's only contractors who call them agents. They call themself recruiters, or more grandly, recruitment consultants. Recruiters can get up to all sorts of tricks to get more of the money a contractor earns. They can even cheat a contractor out of work by various methods like spamming references. So, you must be aware of the all the stunts they can pull. Gerry McLaughlin has seen them all in his career as a contractor and advises newer contractors on how to deal with them.