Why put bread and butter on the table for guests at restaurants?

You provide bread and butter on the table if that's included in your SOP on servicing the customers. This is one way of making them feel at ease while waiting for their food order/entrees coming from the kitchen. It is also one way to minimize boredom for children and the old ones, that will keep them from running around the premise, make noise or complaints from a long wait respectively.

It is comfortable for the guets to have bread and butter on the table, he can apply butter on the bread in relax mode and there wont be any crumble.