If you want your book published by a renowned Traditional publishing house, you WANT a literary agent to represent you. In case of self-publishing, there is no need for an agent. But, it is harder for your book to make it in the hands of an editor without a literary agent. What you should know is that literary agents are professionals in their field and they have up to date contacts and information about the publishing industry. They perform more functions than just getting your book published.
A literary agent can help mould your manuscript and polish it to make it look more attractive to the editor, increasing its chances of getting published. Agents have more knowledge about the publishing industry and know the right editors and what kinds of books are trending. Moreover, a literary agent will try to get you the best deal and will help you with the contracts and the sales vetting procedure.
Publishing contracts are long and technical with many key points that you might not understand on your own. It is important to have an expert who can explain those things to you along with negotiating the best terms.
A literary agent will also undertake the foreign sales and will handle film and TV rights. They will present you with the prospect that is the most profitable, because that is from where they will get the most commission.
A literary agent is not someone you work with, but is also your friend who will guide you for long term and take your career in the right direction. Mostly, agents become just as invested in a book as the writer himself. Writing is not a stable career and a good agent can make a lot of difference in how you make it as a writer.
But there are times when you do need an agent and times when you don’t. Agents take their commission from the sales of the books so they have the most interest in seeing your book become a bestseller. But if you have a poetry book or an anthology, they won’t be interested. If you have written a non-fiction “How to” book about some topic that is not mainstream, you are better off without an agent.
Getting your book noticed by an agent is also a difficult job because agents are flooded with query letters by thousands of authors. But it is worth it to have a good agent on your side.