It goes something like this. Restaurants have to charge a certain amount per meal to stay in business. They chop up this price into 2 sectors. One is the price of the food. One is the tip. This is the staple in the US. To Insure Proper Service is what TIPS stands for. A waitress or waiter can be really shit or really good in an infinite amount of ways. If he or she is good, they make more money. If they are not, they will make less money. The restaurant could just take the extra amount and apply it to the food, but there would be less motivation for the waiter to smile, check on your food after being served, and other things that are considered proper. Without the waiter or waitress, the entire system of Tips goes out the window and a plea to just give us more money is simply being made. I’m seriously at the point where I only pay in cash at places like this.

Also, on your point of the total amount of the bill. Waiters are encouraged to upsell. If they, for example, tell you the specials (usually higher priced), suggest an appetizer, or make a special effort to see that everybody always has an alcoholic drink in front of them and dessert to finish off, the total bill will be much more. That extra work means more revenue for the restaurant and more money for the employee.