Well, maybe it will? There is a raised "spine" on the hatch that may also add some stability in a side wind, and I added the fin mostly for looks, truth be told.
Along with the diffuser fins on the underside, I wanted to make the raison d'être of the car as obvious as possible. The little fin can't really hurt, and it might help; and I can always put a larger fin on there if it is needed.