Why might seclective breeding be used in horses?

To produce a horse with certain qualities. Example: you have a herd of thoroughbred mares and a pair of thoroughbred stallions at your barn. Some mares in your herd are tall, some are medium height, and sOme are super short for a thoroughbred. Say that you want a fast thoroughbred to race. And your taller mares are faster than all the others, and the taller thoroughbred is faster than the shorter one. So you are obviously are going to breed a tall, fast mare with the tall, fast stallion in hopes of producing a foal with this specific quality. (speed)