If you're in a PCC competition or something like that, just use mag pouches on your belt. It's fastest.
If you want to store your FPC in your truck, RV, ATV, boat, as a bug out bag, etc. it's convenient mag storage to have on board the gun to just grab and go.
It's not as tactical as mags in belt pouches or a chest rig for combat... but it's way more convenient than having no spare mags with you.
Plus, if you're willing to do a little practice and figure out a technique that works for you, the mag storage access isn't that unwieldy. Not as fast as from belt pouches, but faster than mags that aren't with you.
Or if you don't want the extra weight at the butt end, just leave the compartments empty.