If you are looking, then are going to try to sell you the positions they have. This can open doors and of course, it's free. But they will never tell you not to take a position.
If you are using a recruiter for a position you have, they want you to hire someone, regardless of how well that person will perform.
Nevertheless, they can automate key parts of looking for a position.
Most of the time if a recruiter does not come to me with a job in mind, it does not work out. I have had lunches with a lot of recruiters who just want to network and none of them have come back to me with an opportunity. The times working with a recruiter has worked, the recruiter had a job already setup, found me online or through my network, and made the connection that way.
Recruiter: 3
Network: 1
Direct: 1