Due to the overwhelming number of writers seeking a contract with a major publisher, editors cannot possibly keep up with the hundreds of unsolicited submissions that come in every week. This is why most major publishers do not accept submissions directly from authors who are seeking publication. Instead, they rely on a network of literary agents who understand the publishing business, have relationships with editors, and know the subject areas and types of books that specific editors are most likely to be interested in. Within each of the major publishing houses, there are dozens of editors, each with their own interests and "wish lists" for books. A literary agent is usually the most efficient way to find the right editor, and the right publisher, for your book. An agent can help you improve your book proposal, resulting in stronger offers from publishers, and consider publishers you may not have even thought of approaching.
Publishing is a very specialized field, and book contracts can be especially complicated. Just like you should never sign a business contract with a lawyer, I don't recommend you sign a book contract without a literary agent. A literary agent can help you secure a better deal by helping you to select the right publisher and negotiating improved contract terms that will protect your financial and intellectual property interests.
I have years of experience negotiating contracts with the major publishing houses, and I will bring this business experience to bear when working on your behalf. You can undermine your own financial and professional success by trying to negotiate a contract on your own. I've seen it happen many times. Often by the time I'm asked to become involved, it's too late.