Talent agencies are organizations that provide representation services for their clients (actors, writers, directors, etc.) in exchange for a percentage of the client's earning.
At the professional tier of filmmaking having repensentation is considered essential. Agents have a level of access and influence that an individual artist would find impossible to achieve. For actors this translates into sides for auditions and an understanding of the complexities of SAG contract negotiations.
Agents are invaluable for writers as well. For one, it can be quite difficult to get a studio or independant producer to read a screenplay that is not submitted to them through an agent.
In essence, when its time to turn pro, landing an agent is a necessity.