Some prostitutes are happy with their work, too. I know it's rude to say this to any American but working for money is prostitution. That is working for money, of course, not playing around and getting money in return. Working for money is when you're spending your life doing something you wouldn't choose to do if not for the inducement of payment. You are selling yourself for money.
All working for money is prostitution. Some are just in better positions than others. High-class prostitutes can fly on private jets to exotic locales where they feast on fine wine and caviar in their minks. Low-class prostitutes... well.. I think people know what their lives can be like. The job is the same, though.
The precise "benefit" in terms of service quality from tipping wait staff is the groveling factor. For some people like myself, it's not something I enjoy being part of.