Why should one reply to an invitation promptly?

Foreign custom is much more strict than Chinese custom in the matter of replying to invitations. When you receive an invitation you should answer it immediately, saying definitely whether you are able to accept it or not. This is because the hostess probably wants to have a certain number of guests at her table, and if you cannot come she will want to ask someone else. And it is not polite to ask anyone at the last minute. So you should let her know as soon as possible whether you can come or not. Sometimes one is invited to a large tea generally in honor of some special person or occasion by written, printed, or engraved invitation cards, ff a great many people are invited, it is not necessary to reply to the invitation unless it bears the letters" R. S. V. P". Whenever the letters "R. S. V. P" appear on an invitation it is absolutely necessary to reply. These letters stand for the French words "Respondez s’il vous plait," which mean, "will you please reply?" Perhaps your hostess needs to know how much room or food or entertainment is required.