In the past couple of weeks I've had writers asking me if the information contained within their queries/synopsis truly makes a difference to an agent or publisher. Well, let me put it this way so you can better understand:
When you send a query along with your synopsis to an agent, that is the information he uses to 'fight' for your place in that publishing house. There's a 'board' meeting where various editors bring in their own suggestions for a few books they want to see in the upcoming release schedule. Your agent must have the information to better equip himself and make the editor he is proposing your manuscript fight just as hard in that meeting.
They look at the demographics of a book, the market for it, its competition, a slant for publicity, etc. All these elements are what a good agent can take away from a query/synopsis if you provide the information he requests in his literary agency's guidelines.