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.
In Part One of my "no curls" arm specialization training series, I talked about the absurdity of most arm specialization routines. I proposed that most trainees do more than enough biceps curls and triceps press downs and don't really need more of either of them.