If you want to be a professional writer, you need an agent.
Few big publishing houses take unagented submissions, and the ones that do leave them languishing for years on the slushpile. So having an agent will get your work read by more editors, at better houses, and more quickly, too.
An agent will also ensure that your contract is fair. So they'll ensure that there is a good reversion clause in place, for example, and perhaps an escalator which increases royalty rates after a certain amount of books are sold.
Agents negotiate your contract up so that you make more money and sell fewer rights. Their commission usually comes to less than the increase in income that they negotiate for you, and so they're well worth having financially, too.
A good agent will sell foreign rights and newspaper serialisation rights wherever possible, so maximising revenue for each book you write.
An agent will rarely represent you if your work is more suited to small presses; or if you write only poetry or short stories. Only books. And each has their own area of expertise, so make sure you submit to the right agent if you're trying for one.
It can be difficult to find an agent, but remember: they depend on representing clients who sell. Agents want to find exciting, interesting new voices whose work has long-term sales potential. If they're as-yet unpublished, so much the better—a new writer has no record of failure.
Finally, an agent is paid through commission on the sales they make for you. If they ask for a retainer or up-front fees, remember Dorothy Deering and run.