I have the guage and the comperator that attaches to my calipers. What it does for me is determines the max length I can make the cartridge from the case head to where the ogive touches the lands.
Now I can make a few cartridges that are just short of the lands and then make a few more a little shorter and then a few more a little shorter in small increments.
Now that I have several different lengths I can see which one shoots the most accurately the same way you experiment with powder charges.
I usually do this length experimentation after I have decided on a powder charge.
The way I understand this is I'm trying to time the bullets exit from the muzzle with the most optimum timing of the barrel harmonics, resulting in the most consistent groups.
Although I believe it does make a more accurate round, it's probably not neccessary to be that accurate for hunting applications.
I just enjoy the process of the quest for the elusive one hole group.
Edited after rereading Nicks post...
I can see the problem if using inconsistent cases,but wouldn't this not be a problem if all the cases were uniform? (ie. all new unfired and flr, or all fired once and neck resized only etc.)