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 probably never thought about it, but most people can't produce as much force using two bilateral limbs to perform an exercise as they can if they perform the exercise with each limb individually and then add together the force of each side. It's called the bilateral deficit and you should take advantage of it to pack on some muscle.