These are for marking/locating the holes for installing a rear spoiler that uses threaded receivers built it to them. Since the holes for the bolts to go through from the underside can’t be correctly located otherwise, they get placed in the holes and then taped to the car when fitting the spoiler so that when the spoiler is removed, they stay and mark the hole locations for drilling.
I last used mine to put the spoiler on my Chrysler 300 C.