The vastus medialis or VMO is the tear-drop shaped muscle on the inside of the knee. Unless specifically targeted in training, it commonly falls behind in strength compared to the well-developed lateral (outside) muscles.
This exercise will keep the VMO from lagging behind. It's something all athletes should be doing at some point during their workouts. Not doing so leaves them vulnerable to knee problems and future injury, like ACL tears.
You may know the name of a particular exercise, but do you know what the agonists in the movement are? How about the synergists? Hrrummpph! And you call yourself a weightlifter!