You`re evening guests will expect some kind of food. It`s the norm when you get invited to a reception. If the wedding reception doesn`t start until around 6pm, I can`t see how you`re going to start the evening one around 8pm. By the time everyones seated, the meal`s eaten, the speeches & toasts done, the chatting, photos etc. If the evening do is in the same place, the staff will also need to clear up after the meal & prepare it for the evening. If there`s entertainment, that will also have to be set up.