both for ease of checking zero at local ranges and the ability to dial spot-on for a short range head shot or whatever without holding under.
but i do have a 6x36 that i had both CDS and LR dots installed in, and the dots require a 200 or 250 yard zero.
in the end, it's just a number on the knob, except a 100 yard zero gives you the ability to dial 100-~700 and the 250 yard zero gives you 250-~750 with the 15 moa you get with one turn of a Leupold knob
