At least one professional thinks your script is worth reading, and not just your Mom. Somebody in your camp is capable of speaking their language. Somebody in your camp is capable of making a deal. Agents make contacts for a living. They talk to studios, production companies, and other agents every day, and they know the business.
Agents are negotiators, deal makers.
I don't even want to try. This is reason number two why you need an agent. In the long run, they have the potential to make your job easier. This does not mean you get to stop working after you get an agent. First and foremost, keep writing.