If you want to sell your book to a major New York publisher, you need an agent, no question. As with anything in life, there are rare exceptions, but do you want to drastically narrow your chances in hopes of winning the lottery? If so, why? A good agent has relationships with editors and publishers, they know who’s looking for what, they know how to sell a book, they know how to negotiate a good contract, and they are skilled at advocating for you during the publishing process – getting blurbs, a good cover, a marketing budget and much, much more.
Yes, they get a 15% commission. But, they almost always earn it, and in most cases, they’re going to get you more money and a better contract, so you’ll end up with more cash in your pocket even after their cut. If anything, I’d say my agent deserves considerably more than what she’s earned by representing me.
Even if your book is a quieter book and you hope to sell to a small publisher, an agent is a very good idea. While some presses will consider un-agented submissions, an agent will give you a better chance and likely get you a better contract.