With servers you are better able to control the flow of food. Also the servers can answer questions about what an item is. Customers at paid events at times feel they have a right to take one of everything offered. Then when they sit down and start eating will only take one bite and find they don't like it the rest is waited. With servers some of this is avoided.
Desserts freeze very well.
That is one of the main reasons I now do a dessert selection on my buffets.