[Benjamin]

A restaurant is different from a shop. Your waiter/waitress is with you throughout the duration of your time there usually attending to your every need to make sure you have an enjoyable meal and evening since you have decided not to cook for yourself that night. They are kind of like your mini butler if you will. If they have given you a lovely evening then yes it's rude not to leave them a little extra seeing as most restaurants don't pay very highly.