I'm not trying to insult anyone, it's just never occurred to me to carry more than one and am trying to understand why people do.
The primary consideration for a back up gun is what happens if, for any reason, your primary is no longer usable.
The practice comes from the days when there were only revolvers, actually even earlier, when all handguns were muzzle loading single shots. A "brace" of pistols was fairly common, giving two shot rapidly, twice as much as someone with only one gun.
Move ahead to the revolver era, and it becomes even more important, especially with single action Cap & ball (which take some time to reload) and still important with cartridge firearms, again, in the days of single actions, and their reload times.
Moving further forward in time, we find the DA revolver, which is significantly faster to reload, even before the advent of speed loaders. However, even with the rapid reload of a DA revolver (by the standards of the era) grabbing a second, loaded revolver was still faster than reloading the one you just shot dry.
Don't know how or why a backup gun became called a "New York Reload" but its been known as such for a long time...
Even today with spare mags, a back up gun has utility for some. What are the things that will put your primary out of action??
#1) Running out of ammo (not as big a concern today with hicap mags and spare mags, but still always a possibility...
#2) mechanical failure; something important breaks. Something a tap,rack, bang drill won't fix...
#3) Losing control / possession of your primary handgun. This is NOT as trivial as you might think. In a real world fight, there are times when your gun might be dropped/slips/ or be knocked from your hand(s). And, since about nobody uses a lanyard anymore, getting your primary gun back could be problematical.
Simply put, if your gun comes out of your hands, you might not be able to get it back and without a backup gun, you are disarmed.
For most of us the "need" for a second gun isn't a big one, but it is an important thing for some people.
Were I knowingly going in harm's way, I would pack a backup.
And, if I KNEW I was going into combat, I'd also take a rifle! And friends!!