The first time I heard about the importance of tips as a french guy, I thought waiter's only income were tips, so in my brain it implied waiters worked for free unless you give them tips !
At tipping establishments where health insurance is offered it's not uncommon for servers to lose all of their paycheck to healthcare and then have to give the company some of their tips to cover the rest.
This is almost true. The minimum wage laws do not apply to waiters, so they are often paid $2-3/hr.
Las Vegas casino hostesses (the ones that bring you "free" drinks while you gamble) work exclusively for tips.
I've heard that at some high-end restaurants, waitstaff actually pay to work tables, since the tips on the high bills are high themselves.