This is a form of NLP and creating a social connection. Think about waiters or waitresses that aren't engaged when they wait on you. They're usually standing away from the table and they're probably leaning away from you. I bet you don't enjoy your meal or the dining experience with a disengaged waiter.

When the waitperson is engaged they're probably leaning towards you when they're talking to you. They want you to have a good dining experience and you'll probably ask which dish you should order if you're undecided. Women seem to ask for suggestions more often then men (no statistical proof, just personal experience) and we enjoy our meal more often with an engaged waitperson.

I don't think most waitpeople intentionally engage you to get more tips. It's because they enjoy what they are doing and want you to enjoy your meal. Bigger tips are just a fringe benefit of the experince.