I carry 2 extra mags for the mag, not the ammo.
because Murphy is a bastard, I want to go home after the incident with a loaded weapon, and I have several friends that carry the same pistols.
I also carry a BUG as a NY reload when possible (attire permitting,) and one reload for it for the same reasons.
Maybe your gun is tested and works fine.
What about your ammo?
I shoot a portion of every box of ammo I carry to make sure, at least as best I can, that the whole box isn't defective.
My cousin got a box of Win Silvertips when they were newer and they hadn't worked out the bugs...he had a whole box with bad primers.
I inspect the ammo to make sure its external dimensions are correct and there are no burrs or obvious defects.
Factory loads could have been stored improperly at the shop, and who knows where its been before it gets to you. I've read of factory loads without flash holes, and personally shot Winchester loads that had out of spec cases that caused malfunctions (was OAL, Win White box .45acp.)
These are just a few examples.
I like to use an OAL gage or field strip my pistol and drop rounds in the chamber by hand to make sure they all chamber correctly. I had a Cor-bon load that had a burr on the case mouth, and I've had GI surplus 5.56 with a misshapen shoulder that would not chamber properly.
A BUG (in a different caliber, dif ammo) and spare mags for your primary to go to in case of a malfunction only make sense.
Regarding mags -
Mags get dropped (when carrying, reloading, mag catches get activated during carry sometimes) and feed lips dented, or floor plates pop off and your mag explodes on the deck, springs give out at bad times...etc
Strange debris, pocket lint, sand, and other crap finds its way into your EDC equipment even when you are careful and are constantly cleaning, maintaining it, and using proper holsters/mag pouches.
You carry long enough and you see strange stuff happen.
If a malfunction occurs my first reaction is to do a Rip-Work, in which case I'm changing the mag because I don't have time in a firefight to diagnose problems. Problems which can occur even with tested and maintained equipment.
So yeah, I could on...but you get the point.
I carry spare mags/reloads and a spare weapon when I can.