In fact, the practice of tipping in restaurants probably emerged from the idea of "drink money" -- the idea being that a server should have a drink (or a tipple) at the customer's expense.
Although tipping is now thought of as a distinctively American practice, it was actually an Old World tradition that did not take hold until after the Civil War. (The whole notion of gifting to "inferiors" did not mesh well with the ideals of democracy, writes James Surowiecki in The New Yorker.)
Tipping periodically causes controversy in countries around the world. It's been the source of social debate, and also a cause of strife between customers and servers.