When we give someone who has served us a tip we are able to do so because the putative employer has removed himself somewhat from the transaction. The tipper, in effect, becomes the employer of the moment, with all the attendant authority the employer would have wielded. The larger the tip, the more clearly the tipper is displaying their ability to exercise the employer's authority over the server.
Tipping results from (as they would say in Hebrew) the natisha of the employer of record from the transaction, leaving the matter to the customer and the service giver. By having the employer recuse himself from the terms of the transaction, society places the relationship between the server and the customer into the domain of gift exchange.