It is there to adjust out parallax. You can google or Youtube and see what it is. For most hunters, if you forget to adjust it or have it set 100-200 yards either way you really aren't going to hit far from where you aim. On the ones I use, the yardage markers on the dial are off and I do the head wiggle thing to manually adjust it when I have the time and am taking a longish shot. 95% of the time it is set to 200 and I don't even think about touching it.