It may well be used as a scratcher now, but that doesn't mean that it was its original purpose.

You would usually only find a cow scratcher on (fairly) low ground and rarely near the edge of a hilltop - cows can be rather clumsy at times.

Sheep would definitely take advantage of it, but you can usually tell if they have because the stone gets very polished and black from the lananin (?) in the wool that rubs off.

See: http://www.themodernantiquarian.com/image.php?image_id=6767 where you can see the shiny edges.