Started with some self defense ammo and thinking it will grow from there as it has to be easier mfg wise to use one size primer at some point.
Actually, it started with loads by Federal and Winchester designed for indoor pistol ranges and advertised as cleaner burning ammo, but that's in comparison to WhiteBox and American Eagle loads. I've ran into some SPEER cases with small primer pockets as well. I have yet to see a defense load that uses a small primer.
I discard it and store it separately as I have no shellholders designed to prime with them. I prime off press with a LEE Autoprime when I load on my REDDING Boss single stage and on press when I load with the LEE Classic Turret. Maybe I've overlooked them but I haven't seen a shellholder for either designed for small primer use.