Recruiters are always networking and looking for leads. Candidates are primarily a resource - they are a potential fill for a customer need, or they are a source of leads for other good candidates or potential jobs to fill that the recruiter might not know about. If they call a company and talk to a management professional, even if there are no openings needing a recruiter, they are going to pump that manager about others they might know, and what they've heard of in their network, and will even try to see if that person is interested in looking at positions - they would be mining for both leads on openings and more candidates on those calls.
I'm going to disagree with another answerer who stated that this was mainly so they don't put you in for a position you are already looking at - No reputable recruiter should ever submit your information to any company without your knowledge of who that company is. They may keep it close to the vest, at first, but when you give your permission to be submitted, they should tell you who it is, at that time. So, really, this is mainly about trying to get more leads, first and foremost.
They are mainly looking to an "in" with another company, in general (even if they don't land an agreement for that position, they have made contact with a company that might have needs in their area of specialization, going forward).
Having said that, there's nothing wrong with them networking this way, but there's also no particular obligation on your part to give them names of other professionals you know, or about positions you are aware of.
(I used to recruit with, at that time, a franchise of the nation's largest recruiting company, and this was part of their training).