Tipping is part of an overall pricing strategy that restaurants have settled into because it works for them. Service is an important part of a restaurant patron’s overall experience, but it’s difficult for the restaurant to monitor service. You could put a manager in charge of watching the employees, but it’s difficult to evaluate service from across the room, it’s costly, and it can lead to confrontations in front of patrons. A system with tipping replaces this direct supervision with a financial incentive to provide good service. A waiter (or busboy or sommelier, etc.) who provides a better experience makes more money and the restaurant has happier customers and does better. Economists call such a system “incentive compatible.”
But it takes one more thing for a system with tipping to work. Patrons have to be willing to tip, even if they are never going to return to a restaurant. In the U.S., restaurant patrons have been willing to do this, so the tipping system works here. If this is not the case, as it may not be elsewhere, tipping would not work, so the pricing strategy that works may be different.
People who have waited tables for more than a while can forecast their tips because they know their service and the prices at the place where they work. You probably weren't the only unhappy customer at that pizza joint. The difference between 15-20% and 10% adds up. That waiter will either improve, accept lower earnings, or find something else to do. If the pizza joint pools tips, other employees will figure out who is causing the decline in the pool and do something.