Having an agent greatly increases the likelihood that you will be published. For one thing, on the procedural level, established agents can usually obtain relatively rapid (and serious) consideration for their clients. One basic reason for this is that editors view agents as a valuable screening mechanism— that is, when a project crosses the editor’s desk under an agent’s letterhead, the editor knows it’s undergone vetting from someone in the industry who is familiar with the applicable standards of quality and market considerations
Most major publishing houses claim to have policies that prevent them from even considering unagented/unsolicited submissions. “Unagented” means that a literary agent did not make the submission. “Unsolicited” means that no one at the publisher asked for the submission.
Imagine that you’re an acquisitions editor at one of America’s largest publishing firms in New York City. You have a master’s degree from an Ivy League college and you, at least, think you’re smarter than most other people. Yet you’re earning a lot less money than most of the people who graduated with you. Your classmates have become lawyers, accountants, bankers, and so forth, and they all seem to own large, well-appointed apartments or homes—whereas you if you fall out of bed, might land in the bathtub of your minuscule New York flat.