Recruiters are established in their markets and have access to a large talent pool of relevant candidates. They then pre-screen and reference them, leaving a shortlist of a very high quality.
Many recruitment companies put work into creating an approachable online presence. This includes having an established website where in turn, candidates regularly submit their details. This means the leads recruiters have are strong and often individuals who are keen to make a move.
Looking at it from the other perspective too, a recruiters skills can be extremely valuable and help you prevent hiring any bad candidates. They are always in the know and are often aware of who is looking for work, how capable they are, and salary expectations.