how can agents help actors?

Answer 1

Once you have developed your craft as an actor, an agent can submit you for higher level work. Studio films send out casting notices only to agents, who submit actors for the work. The same is true for theater actors in places like New York and London.

Answer 2

They do all of the "left brain" nasties for you like knocking on doors, sending resumes, and bugging people for your checks who are behind in payment. If you are one of the rare artists who can handle the artistic AND the business side of your art you don't need an agent. If you HATE confrontation and you don't like the pavement pounding, banging on doors promotional aspects of the arts you might want to consider an agent.