A hunting buddy of mine ask a question about that to day and when I answered him we had to go to the range and shoot before he really understood.
He normally "0" at about 40 yards and he had trouble understanding why I "0" at 150 yds.
The difference is when a shot turns up at a longer than normal range the capability of the gun and the shooter are both maximized. I am only 2 inches high at the top of my trajectory, not enough to cause a miss, while the way he was sighting in he would be as much as a foot below the deer I would aim straight at and take with out a problem at 175 yds.
Now I know that the normally deer shot is not that long, I also know that on the logging roads in the woods around here that 200 yd shots turn up occasionally. When you have any open spots at all it is best to be prepared.
When we shot at a deer target at 150 yds he had a nice group 3-4 inches below the deers chest several were in the knee.
None of us want a deer hobbling away on 3 legs!
Sighting in for what we call point blank range gives you the best answer for yourself and your guns capabilities. And the confidence needed to make a good shot.