Perennial topic. The same reasons (and debates) get brought out and trotted around the corral each time.
Remember when a pistol might come with only 1 magazine? Then it might come with a spare magazine (which was a big deal for some advertising)? Nowadays, we can see gun companies advertising a couple spare mags, and sometimes a special package might come along where more spare mags might be included.
https://www.ammoland.com/2021/01/smith-wesson-mp9-shield-9mm-handgun-bundle/#axzz6lRU3Syod
The cause? Market forces.
Now, for LE/Gov, there's policy and practice to drive such things.
For example, at my former agency we had a policy (probably changed again by now) that required people in uniform assignments to carry 2 spare magazines. More could be carried, if desired. That was a continuation of when we were required, in the days of service revolvers, to carry 12 spare rounds of ammunition. People assigned to plainclothes were required to carry at least a single spare mag, but could carry more. There were some variations, such as when a couple of the available issue weapons used single stack mags, and either 3 or 4 spare mags could be issued.
When I was assigned to a plainclothes position for the last half of my career, I typically carried a single spare mag. Sometimes more, depending on my case load and any specific anticipated activities of the day or night (warrant service, etc). Sure, I carried more in my unmarked veh, as well as spare (boxed) ammo much of the time. Back in my service revolver days I often carried more than the minimum spare 12 rounds, too, especially when working areas where my cover might be up to an hour or more distant.
Nowadays, in my retirement, I continue the same practice of carrying a spare magazine, or a speedstrip (or speedloader). The single spare mag is regardless of whether it's a single or double stack mag. Every once in a while, depending on my planned activities and where I'm going to be, I might add a second spare mag (or toss 1-2 in my veh) ... or more speedstrips/speedloaders.
Now, an exception (always is
) to my normal practice is when I carry one of my LCP's. I seldom carry a spare mag for the LCP I'm carrying. I have my own reasons for this, and it's not like I haven't considered the advantages and disadvantages of both the choice of the LCP, and whether or not to carry an extra mag for it.
Having carried a concealed weapon with a badge/ID or one sort or another for 38 years, and having served (and received training) as a LE firearms instructor for many of those years, I feel justified in making such assessments and decisions for myself. I don't presume to make them for others, though.