In many "run of the mill" diners, restaurants, etc, (where normal folk go), wait staff depend on tips since they are normally paid a rate less than minimum wage. Typically, tips are in the 15% range.
Where there are separate bodies bussing the tables, it is typical for the wait staff to share some of the tips with them.
Tips are reported as income, either by the person, or by the employer (and this is by law, in some cases). An employer can be audited and a wait staff's pay allotted a certain percentage of sales as "tip income" This also figures in the calculations as to the amount of social security taxes paid/withheld ...
That's from a bookkeepping point of view.
Tips are always welcome income. I am fortunate that most of the time I received good service, worth evert cent of the tip I leave.
Then there are the drive-by places that have a cup for "tips". At an ice cream place we go to from time to time, most are college students so the tip cup is marked "Help Support Hungry COllege Students" . Since the service anyway is pleasantly friendly, I always leave something.