Unlike the other responses, I will provide a more mundane example that's hopefully more relatable: Giving your mailman or deliveryman a can of coke/pepsi as a token of appreciation.

Next time, they're going to be more careful and timely handling your package, and you'll keep asking if they want a drink. It's unfair, because you will get special treatment compared to those who don't, but we're all human and everyone's happier. This is legal bribery.