Reaming is different than uniforming. Uniforming insures the primer pocket is set to a repeatable depth on each piece of brass, and is used in match ammo prep. I find it also does a great job in cleaning out primer pockets in subsequent reloads. It takes no effort, just a gently twist and gets everything out. A good quality uniformer does not take off any brass after the first use and does not enlarge the primer pockets.

This also helps make sure the primer can be seated to full depth and helps avoid high primers which can be very dangerous in some kinds of actions. (Say a break action where closing the gun could set off a high primer, or a revolver where a high primer could hang up the cylinder).

I use a uniformer AFTER I ream out the crimp of military brass.