Why do we tip?

Link to NY Times article

Economists have struggled to explain tipping. Why tip at all, since
the bill is presented at the end of a meal and can't retroactively
improve service? And certainly there's no reason to tip at a
restaurant you will never revisit.

"The need to pay, psychologically, for the guilt involved in the
unequal relationship is so strong that very few are able to ignore
it." Ego needs also play a part, especially when it comes to
overtipping, according to the Israeli social psychologist Boas Shamir.
Ego needs also play a part, especially when it comes to overtipping,
according to the Israeli social psychologist Boas Shamir.