Liberty Sanders
Master
One reason is strategic.
I carry a brace of Smith N frames, one straight draw and one cross draw.
In a carjacking attempt a crossdraw is easier to get to than a straight draw. It can also be drawn backhand with the weak hand should the strong hand be unavailable, such as a bad guy attacking suddenly from your strong side.
Heck, I forgot...back when I was doing a lot of traveling for the NRA I always carried two guns, which was when I developed the habit.
If you get involved in a shooting the police are taking your gun. No, this is not an anti-gun conspiracy. It's evidence and has to be processed. Chances are you'll have no opportunity to get it back until after a grand jury hearing or other legal process.
If you had one gun, you now have NO gun. This will be especially troublesome if you're far from home.
Of course if you do the Wild West thing and use both of them this advantage is negated.