The answer to this question depends upon why your are asking:
If you are a restaurant owner, the main advantage is the ability to save on the cost of labor by eliminating some of the duties of a waiter. It is also possible for the cooks to prep the ingredients early in the day and cook in large batches as dishes need to be refilled. Cooking in large batches takes less time and energy than cooking each dish to order. They also have a set menu, which cuts down on the time a chef or cook needs to learn new dishes and train the kitchen staff to prepare them.
The main disadvantage is the safety issue in storing both raw ingredients and pre-prepared food, and also maintaining a safe temperature in the service area. Hot and cold foods have a different range of safe temperatures, and it is usually a very narrow margin. You not only need the appropriate equipment, but you need a responsible staff who will check the temperatures regularly and discard any and all foods that have become unsafe.
If you are considering holding an event and want to determine whether or not to serve the food banquet style, you need to take into consideration that a sit-down meal requires a larger space and, if your guests are not going to serve themselves, the cost of hiring waiters is considerable.
If you are having a more casual affair and want to give your guests an opportunity to mingle; or if you are on a limited budget, banquet service will be the better option.
We have published What are the advantages and disadvantages of banquet service in food and beverage? on 18/08/2013 and his number is 17408