it works in an ideal situation. which is to say, it won't work exactly right anywhere else. it assumes everything is 100% -- your bearing edges, the shell roundness, the consistency of the head, etc etc.
HOWEVER there IS a use for it in nonideal circumstances:
tune your drums by ear.
use the drum dial to see what values are at each lug. write them down.
if you bust a head at a show, tune the head quickly using those values. voila, your head is in tune, assuming the tuning situation hasn't changed (which, if you take care of your drums, it shouldn't).