Recruiters (the good ones) spend significant time finding candidates and putting them in front of employers. They have relationships with the employers, sometimes exclusive placement deals. After an unsuccessful interview the employer usually (in my experience) talks to the recruiter about why the candidate was not suitable so neither party wastes time -- the recruiter can use that information to send candidates more likely to get hired, which is the only way recruiters get paid.

Recruiters will put a candidate to the bottom of the pile if they flunk multiple interviews, or badly bomb one. That embarrasses the recruiter -- they lose credibility with their actual customer (the employer) if they keep sending unqualified candidates and idiots who show up for interviews late in gym shorts and a Metallica t-shirt who don't know what the company they're interviewing at does (actual experience I had). A good recruiter will tell you how to do better next time. They probably won't send you back to the same employer for another try right away, though. A good recruiter can prepare candidates for the interview, the bad ones just throw mud at the wall to see what sticks.