My thought on deburring the flash holes is that the flash of the primer ignites the powder more evenly with an even hole that is the same depth on all of the cases making things more consistent. The reasoning for squaring and cutting the primer pockets to an even depth is that the primers can be seated to the same depth and flat in the pocket. So that the firing pin will strike it with a consistent pressure and the primer doesn't move forward so it then ignites consistently kind of like some of the bench rest guys used to fill the space in their case with fiber so that the powder didn't flow to the side of the case, or they would use a powder that they could over fill the case and have a compressed load for an even ignition of the powder instead of igniting one side of the load first. It's all in how one thinks about it may be kind of like being superstitious and wearing a lucky shirt or carrying a lucky call. Removing things that might or might not make a difference in accuracy at distance. Besides it's fun to me.