You can send queries directly to publishers and they will go into their slush pile. Sometimes a publisher will pluck a winning book from this endless pile. But that happens rarely.
By making a query to an agent you are adding at least one more step to the lengthy and complicated process--but you are making your job easier and improving your odds of success.
Agents screen queries thoroughly, request additional information, suggest changes, provide some advice and encouragement (at least this agency does), all before agreeing to represent an author. By the time an offer to represent is made your manuscript will have been reviewed by a professional editor, the genre will have been researched and the agent will believe that your manuscript can be marketed successfully.
Agents read trade journals so that they know who is buying what, what is being sought and what is currently not being published. A good agent knows the markets, knows which house is publishing what and, in addition, has made some personal contacts who trust their judgment and are willing to read recommended manuscripts.
The bottom line is that the manuscript must be good in order to be accepted for publication. Your agent will help you to refine the product, assist you in making your appeal more professional and encourage you along the way.
Once accepted by a publishing house the agent negotiates the contract, including advances, time-frame, possible marketing, and other requirements. Your agent will also receive and disburse advances and royalties.
Your agent will also help you with additional products and can provide you with ideas.
You and your agent are a team and MUST work together. During the process of considering a new client we will be looking for signs that suggest that we will be able to work together, that the new client can accept advice and criticism and that we can each expect the other to participate fully in the collaboration. If there is something about the approach, philosophy or attitude of the agent or his or her representative that you, as an author do not like, back out early. We are professionals and expect the same of our clients.
So the answer is, NO, you do not NEED an agent, but an agent will improve your chances of success and make it easier to get there.
The Client Development Team
Rebecca Pratt Literary Group