It's a cultural norm that would persist (and does) regardless of whether or not there is a genuine purpose to it. That some restaurant owners use it to avoid pa

It's a cultural norm that would persist (and does) regardless of whether or not there is a genuine purpose to it. That some restaurant owners use it to avoid paying workers doesn't mean that's "its" purpose. Others use it to supplement revenue (like when Starbucks and Uber and those places just keep the tips for corporate). If it has any purpose at all, it's a social signaling protocol to say "I am a good person" that servers use guilt to amplify as much as possible.