What does sparring or fighting do?

Sparring helps teach you timing and how to make adjustments for a moving, thinking, opponent that is probably trying to do to you what you are trying to do to them. It also helps teach you what adjustments to make based on the size of your opponent if you spare with a variety of people of different sizes and height as well. You also learn how they think, move, and react in relation to you, giving you a better understanding of fighting in general.