It’s functionally a way to communicate happiness with the service.
The restaurants I am a regular at know if I don’t leave a fat tip I wasn’t happy with how they performed. Waiting 20+ minutes just to pay is unacceptable to give a recent example. They were understaffed and some old dude In the attached hotel insisted on print out copies of his reservation details that he then argued about. I could have paid, cash or card, and been out in under a minute while this dude was reading his papers but instead I just sat there for almost half an hour after finishing my meal.
Should they still get paid unlike in the American system? Yes. But I’m fine with tipping as a general concept. In Germany we call it Trinkgeld and it’s usually 10%, and not exactly a thing you are expected to do every time you eat out, but I usually do 20%+ if I was satisfied.