One is to assure the guests that they have accomodations by the time they arrive at the hotel. Two, so the room/rooms will be ready and available. Three, so as to avoid confusion when the guests arrive to find there is no room available. Four, so that the hotel management will know how many rooms are still available for occupation. Five, so that the chef and kitchen staff will know how many guests are arriving and be able to stock on food and drinks.