How can we tell s/he has been making progress?
Rating is the only objective criteria. Your teacher's comments are definitely subjective and can't compare. Judging by seeing your child winning or losing games can be misleading because we can't track them personally. We may have bias and only remember what we want to remember. Only rating is a cold number. It's not absolutely accurate, but it draws a rough picture.
You child need such an objective target to motivate him working hard. Achieving goals also provides satisfaction and positive feedback. S/he targets higher next time.