cargo hit the nail on the head....
for those hunting with a heavier arrow and average speeds, the difference between guessing a deer at 28 yards and actually having it at 33, plus the smaller window on the kill zone if you' re 22 ft up in a tree...
And your 30 yard group of 3-4 inches suddenly seems inadequate.
I' ll also voice agreement with ijimmy... the rangefinders that adjust for level (I think pentax has just come out with one too) clearly offer an advantage in effective accuracy.
That can also be huge out west (I am guilty of missing a chip-shot on a huge muley that was as much down as he was away)...