There can be quite a few pros.
For one, the menu can open up. Things they wouldn't do normally on the first meeting, they'd be more ok with. COF is still the most elusive thing for me, and aside from the few known suspects, I can only pull this off with a few with whom I have built that rapport with.
Another is out-of-agency meetings which can even mean more convenient scheduling. Prices won't be any better, but it's better for them if they're keeping it all opposed to paying someone else.
Then there is just good ol' comfort. I love the hunt for something new, but that can lead to some disappointments. Sometimes, you know what you want and you know who will give it to you. Nothing wrong with that. I'll usually double back as a regular after a miss. Just to level myself out.
Last is "dating" them. Different from the second point above. This is rare. At the end of the day, they want to be paid. But there have been a few girls I've casually hooked up with outside of the work environment after food and drinks.