Why is it essential to refer to the customer as a guest?
It changes the way you and your staff think about the transaction.
When you have someone over at your home, they aren't customers; they are a guest. You are providing them with hospitality.
This is the same in your restaurant. Everyone who dines in your restaurant is a guest. Yes, they are there to satisfy the basic need of hunger, but they chose your full-service restaurant because they want to be taken care of; they are looking for an experience.