What is gained by learning something and never doing it again? We're constantly learning new things by building on what we already know, and those things we enjoy, we like to do more than once. Kids love repetition. (How many times have you been asked to read the same bedtime story?)
Review pieces are great for mastering new techniques, since you already know the notes and don't have to think about that part of playing.
Natural learning takes place through repetition. Through repetition, sensory-motor learning takes place. This kind of learning is necessary for students to learn more advanced techniques.