Many people do book 3 nights if they only want 2 - if you are catching a late flight, paying for an extra night would allow you to stay in your room until early evening. This is especially common in, say, India when the flights back to the UK all leave at midnight.

Some people also book a room the night before they arrive so that - if they are on a flight that lands at 6am - they can go to the hotel for 7am and be certain to have a room available. This one needs arranging with the hotel beforehand, however, so they don't resell your room.