What I carry on a given day depends on multiple factors, such as:
* Function (where I'm going)
* Attire (what I'm wearing)
* Risk (what are the potential risks where I'm headed)
* Belief system (how I "feel" about it)
Quite often, the last one can be the deciding factor, everything else considered. I was taught to "listen to that little bird on your shoulder", and to date it has not yet let me down. Therefore, I recommend it highly.
The "why" seems to be easy to define, IMO. Generally speaking, there's simply no faster reload than grabbing a second gun. One can still be firing from their primary weapon WHILE bringing a second weapon into the fray. One simply can't accomplish that with a single gun, no matter how adept their mag reload skills.
I own multiple holsters for each handgun I own, so I can adapt as readily as possible to what I believe I should be carrying on a given day. And l've carried as many as 4, though that was rare.
And no, it's not all that difficult, IMO. I've found carrying two to be quite convenient, and usually with reloads. Probably most often throughout my life, though, I've carried 1 and some reloads.
Though I'm not a large guy (5'10", 155 lbs.), I've found little trouble carrying large handguns & reloads (mags or speedloaders), or multiple handguns, so I do.
In colder weather I wear sweaters a lot, so I wear shoulder rigs more, to facilitate drawing. Lifting up a winter coat CAN present some issues, and wearing a coat and a sweater can become an exponential hindrance if I'm wearing a belt rig. As a motorcyclist, I've found shoulder rigs to be beneficial as well, since I'm always wearing some sort of riding jacket.
I don't really do the "flannel shirt tucked in the jeans" thing, so unless fairly formally dressed (as when wearing a suit), my shirts / sweaters tend to hang out. Flannel or western shirts tucked in simply don't fit my "look" or my "known image". Kudos to those of you who can wear that, of course, but I can't.
ANYTHING that "doesn't look right" or "out of image" COULD draw attention, so I avoid that to the extent possible. I rarely open carry (though perfectly acceptable here in Indiana) for the same reason. I prefer to look innocuous, like someone who wouldn't possibly carry a gun. I don't want an "easy mark" image, but I don't want a "tough guy" image, either. Just "Joe Average American". Innocuous.
In more favorable weather, I tend to carry via belt rigs more frequently. In this case, most often 1 gun and 1 mag or 2 speedloaders.
If more than one gun, small of back is my preferred choice more often than a second gun in a cross-draw holster, though I can do that. I've never really adapted to ankle rigs, so I tend not to bother with them, though there's some advantages to using them for those that like 'em.
Most of my shoulder rigs are "convertible", so it's easy to substitute the double clip pouch for a second holster. If I feel the need for 3 guns, simply tuck the 3rd in a small of back holster.
WHY would I "need" to carry 2 or 3 guns, plus (perhaps) reloads, as a civilian? Don't know that I ever do. Didn't know that I needed to when I was in that line of work, either. But I do, always, listen to that "little bird on my shoulder". And if he says "take 3", that's what I take. Most often not.
Then again, wasn't too long ago when some scumbag walked in to a mall in Chicago area and shot up a woman's clothing store (Lane Bryant, if I remember correctly), killed a woman, wounded some others, and fled before the cops could even be called, let alone arrive. I don't want to be caught in the middle of that stupidity, either.
Should I NEED to carry more than 1 gun? NO! By gawd, I SHOULDN'T NEED to carry a gun at all! But until we figure out how to get guns away from ALL the bad guys, drug dealers, gang-bangers, serial killers, random psychos, thugs, lunatics and crazies out there, I think I'll just keep listening to that "little bird".
Ya just never know.