1. Getting you on a call (anyone, not just recruiter) is a way to drive the interaction to a conclusion, one way or the other.
2. Getting you on a call gives them the opportunity to read you.
3. Getting you on a call gives them the opportunity (if they are of the type), to make you feel a social obligation to reciprocate (information, time, etc)
I'd add sheer information gathering: even if the call leads to nowhere, they get a sense of how it's going in your company, salary data matched to a role, and a reality check on what they anticipated vs your actual situation.
I think it can be intesting to talk to a recruiter even if they have nothing decent to offer, provided one also enjoys just talking to other people. I just don't think it's an optimal path for anyone actually interested in a position, doing one's homework and applying to a company one's actually interested in can have better results.
