Generally, publishers don’t just accept submissions from any aspiring author. They go through agents. So you must first convince an agent that you will be a lucrative investment. As a rule, agents take 15% of everything you make from a book.
Speaking as an author, I love my agent. My agent is on commission, on my side. A good accountant is supposed to save you more money than they cost you; a good agent will actually make you more money. The agent's job is to match your book with the right editor, or if there's more than one who's likely to buy it they run an auction: once they've got you a publisher they then squeeze as much money out of the publisher as possible -- publishers always low-ball the advance and terms they offer an unrepresented author -- and then vet the contract small-print for landmines. A survey an author of my acquaintance conducted about five years ago came up with statistics for agented vs. un-agented books in the SF/F field (which is what I write), polling professional authors on a mailing list; in general, the book advances of authors who go through agents are between 150% and 200% of those received by unagented authors.
The benefits of an agent are likely to be lower for a non-fic author who writes technical/business books on an irregular basis -- but still reasonable. But for a fiction author who eats or starves on the basis of pushing out at least one book a year, the agent's ability to market the author and push their career track is essential.
They are, after all, on commission. And the more they can earn for the author, the more they get to take home too.