Stockpiling is critical to insure when the mill is fired up, it will have a steady flow of ore for production during the ebb and flow of the mining.   Also some of lower grade ore will be segregated for mill ramp-up to be sure they don't throw the baby out with the bath water while learning the right recipe on how to cook the metals from the ore.   This is especially true of PMGs.   No expert - just from experience in other investments.   So take it from whence it came but much of it makes sense once you hear it enough times.