its essentially for setting/determining the proper bullet depth. Proper bullet depth will vary from gun to gun regardless of caliber. For the best accuracy, the bullet should be seated just off the rifling. I believe that the bullet relationship to the throat/rifling is called the "lead". (As in Leading a horse.) Its essentially how far the bullet moves before contacting the rifling.Too long a OAL and the bullet will sit against the rifling, causing pressures to increase when fired. To short and it will affect accuracy, and possibly erode the throat quicker. Bench shooters are VERY picky, and the lead will be spot on every time. This quite often only works though if your loading a single cartridge at a time in your gun. For instance, on my 700's, if I set the lead to where I really WANT, then the cartridge is too long for the box mag, so most times the lead dimension is a compromise, at least for hunters. hope this helps.
