I shot "benchrest" in the NBRSA for many years. It is my understanding that the square has no official purpose, that said, it is used to judge mirage and wind drift. That may have been the original purpose. I have seen conditions when the "mothball" was invisible in mirage or rain.
Things may have changed, but probably 2/3 of the shooters I know did not use a dot in their scope because it covered too large an area on the target. Most competitors liked a POI below the X to avoid the black of the box and not shoot the point of aim.
jerrold