In the context of my LE job, any handgun other than the one in the duty holster, at 0300, was a “Back-Up Gun.” During personal time, before retirement, and then, since retirement, it is really just a matter of carrying more than one weapon, largely for accessibility options, as some body parts are not as strong, or as functional, as they used to be.
To elaborate a bit, and present an example, when I awakened this morning, I had to massage and stretch my right thumb/hand/wrist, for some time, to get them working properly. At that moment in time, had I needed to use a handgun, I would have had to use it left-handed.
I started carrying at 0300, in 1984, as a young police rookie. Though not really right-handed, I was right-armed, meaning I throw righty, and write lefty. Drawing the relatively heavy L-Frame from the then-mandated low-slung duty rig was not unlike throwing underhanded. I knew I would be patrolling alone, in a one-man car, so my right hip would be more accessible as I rolled up to scenes. To this day, my right hip is more-accessible, if I were to be car-jacked. It still makes sense to carry on the right side.
One problem with right-hand shooting, however, is that I no longer trust my right thumb, fingers, and wrist to stabilize the frame of some autos, to ensure reliable functioning, and, holding onto a large-gripped weapon, in a scuffle, is NOT as sure as holding onto a small-gripped weapon. So, it is a best practice, for me, to carry revolvers, with relatively small grips, on my right side.
As a police officer, it seemed wise to carry second guns, for the obvious reasons. It made sense to practice lefty, with these second guns, as well as with my primary guns. (One’s weapon hand can become disabled, or otherwise taken out of action.) Being left-handed meant that shooting lefty was not a handicap. I could scale-back now, in retirement, from toting the second gun, but with my right-hand draw being most intuitive, and my left hand being my most-able hand, it seems logical to continue to have a weapon situated for each hand, as before, but the larger weapon, if there is a size difference, being carried lefty, and auto-loaders are best carried lefty.