Gratuities are meant to solve an economic problem. When a person is paid a flat wage for his work, he has no incentive to work to his full capacity. As Peter says in Office Space, the threat of getting canned "will make you work just hard enough not to get fired." Gratuities in service jobs ensure that workers will provide excellent services in order to receive a generous gratuity. The same can be said of bonuses, stock options, piece work, and various other forms of compensation.
When are gratuities a lousy form of compensation? When the quality of the work is largely not affected by the worker's effort. Case in point: pizza delivery. No matter how much effort the delivery person puts into getting the pizza on time, there is no effort that can overcome traffic, speed limits, stop signs, weather, or the main reason pizzas are late, a slow kitchen.
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Yesterday I ordered pizza. It was slightly late, and the charge was slightly higher than stated on the phone for some unidentifiable reason. I tipped $1.50, and the delivery man got angry at me. My simple response is this. Don't be angry at me if you're not getting paid enough. Get angry at your employer. There is no fucking reason they should advertise free delivery if I have to pay the delivery man his salary. A $1.50 tip is sufficient for walking or biking 6 street blocks. He didn't even have to walk up or down stairs.
The insanity of gratuities extends well beyond food delivery. Is a bottle of beer somehow opened better if I tip the bartender a dollar? Will a hotel doorman be able to hold the door longer if I slip him a few dollars?
The real reason many of these jobs are paid gratuities is a disguised form of pricing. By removing the delivery charge but letting people tip, you ask people to give extra money not for the pizza or the beer, but for the poor helpless man standing at your door or the attractive person behind the counter. Instead of charging $7 for a beer, you can charge $6 and ask people to tip. By framing it this way, it doesn't look quite as expensive to John Q. Public, but it's effectively the same bottom line to your company.
It may seem like a clever gimmick, but economically this is counterproductive. Individuals are risk averse. Workers would like to be paid a fixed salary for their work. If you pay them a variable compensation, they will demand higher pay ultimately. So in the pizza case, it is Dominos that loses. Bars could pocket more of the customers' money if they paid bartenders directly. Hotels could charge a flat rate including fees for doorman and bell captain. And yet, few places offer such pricing. So, we just have to suffer through paying these nonsensical gratuities.