Personally I don't do this- I sight in dead-on at 100yds. I think the reason they do this is that it's sort of better, I guess, to miss higher on a deer than lower plus it helps with distance. Obviously, thanks to gravity, our bullets would eventually fall to the ground if nothing stopped them (i.e. a tree, ground, animal, etc...) So the farther the shot, the more the bullet will drop thus creating the reason to zero in a little high of bullseye at the range.
anyone, please correct me if I'm wrong....
...and on that day when my strength is failing, the end draws near and my times has come, still my soul will sing Your praise unending, ten thousand years and then forevermore!