If something is submitted via an agent then the editor can be reasonably sure that both it and the author been vetted and are not completely unworkable. Publishers trust agents in a manner they will never trust the general public. I tried for years to have a novel accepted by a publisher. But with BattleAxe I was so determined to give it every possible chance I thought I'd try a literary agent. I love agents.
Agents are good.
Agents are the buffer between you and the publisher. Agents are someone you can ring up and cry on their shoulder about the sick cat, the comma that won't go where it should, and the boyfriend who won't stay where he should. Agents know what's going on and will do their very best for you because, in the end, they don't earn until you do. But agents are almost as impossible to obtain as publishers. The massive pile of unsolicited manuscripts that end up on tables and in cabinets and on the floors of publishing firms.