A literary agent will go through thousands of books per year and pick the ones they feel have the greatest chance of success at major and minor publishing houses. Essentially, a literary agent is a salesman whose main product is YOU and your book!
At this year's Romantic Times convention, I interviewed the New York Times Bestselling romance and mystery author Jennifer Ashley, who's published over 30 books. Romance readers, in particular, are especially voracious, and it's a challenge for writers to keep up with their demand (they easily can want to read up to four novels per year by the same author!) I asked her if she had any writing ideas which have helped to become so prolific.