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....