An Acting Agent finds jobs for the Actors they represent. They do this by submitting their clients for auditions, pitch them for roles, follow-up on submissions (and subsequent auditions), negotiate for contracts (this includes everything from better pay to billing to working parameters), and renegotiate existing contracts.
While Acting Agents are incredibly valuable, they’re not always necessary. When you’re starting out in the acting field, your focus should be more on procuring experience and growing your resumé. It’s unlikely that an Agent will sign an Actor without much work in their background, making it pretty hard to get an Agent right out of the gate.
Over time, as more jobs begin to amass, you’ll want to look for an Agent to aid in booking auditions and seeking out casting calls for bigger opportunities. So in short, Agents become more necessary as an Actor accumulates work.