Weird. I have never seen that before. I know what it would do, but not why you would need it.
It goes in the 2nd hole in the skimmer that could be plumbed to the floor or an equilizer line. If the skimmer clogged or you made the float diverter force suction from the floor, then it would have to pull up on this check valve. When there was not enough suction , this would be closed. But, I really do not understand why it would be needed.