There's been a pretty mysterious phenomenon I've noticed at conferences, happening twice.

A girl comes alone, initiates a conversation, and asks what you're doing.
As the chat progresses, one says they just graduated not long ago,
another says they've been working in Web2 before.
Then you'll find that they almost always go and add people at each table,
and naturally ask, "Where are you heading next? Can I join you?"
After you take her to the next venue, she disappears not long after,
and when you look again, she's already followed another group to the next spot.
I'm quite curious—what do they usually do in these situations?
Networking? Looking for resources? Or some other kind of tactic?
Can someone who understands explain this?