Sales and marketing go hand and hand. Marketing attracts the customer to your offer and sales, sells it.
So your servers, although they perform an essential part in the restaurant's operations, should be considered part of the marketing team.
Are you teaching your staff to take orders, or are you teaching them to sell? This doesn't just mean they sold another bottle of wine or upgraded to a lobster tail add-on, but are they selling the next visit?
Think about it; the guest is already in the building, they are already going to buy from you. So why not think about creating the best current experience to try and sell the next one? It's great if the guest says they enjoyed the experience as they walk out the door, but it's even better if they say, " I can't wait to come back."