I do, for several reasons.
-Access.
I haven't found a holster/carry location that I can always easily access a gun from. If it's easy to get at when standing/walking, it's hard to get when sitting. Likewise, the best holster I've found for sitting or driving is terrible when standing up.
I can give more examples, but the point is, every location/holster has situations when it's not very good at all. By having more than one gun, I hope to be able to access ONE in a reasonable time.
I think if most people paid close attention over the course of any given day, they would find plenty of times when their chosen carry method doesn't work well.
-Guns fail.
I am sure most people carry a gun they trust, but they are still mechanical devices. While I trust some more than others, I don't feel any are 100% failure proof.
I made my living maintaining big airplanes. They have double, triple, and quad redundant systems with heavily tested, over-engineered parts. I still had plenty to do every night.
Things break or otherwise fail. That's all there is to it.
-Loss of use of one hand or arm.
If I need a gun, the chances of me being injured by the time I need it are pretty good. Some injuries would make it awful hard to operate some guns. The most common examples are hand/wrist/arm injuries leading to semiauto malfunctions, but there are many others. It wouldn't take a huge arm/hand injury in the scheme of things to prevent me from drawing the gun at all. Or even reaching it.
And it might not take much of an injury. If you've ever had even a small back injury you know how much that can limit your movement.
Another gun in another place might be the only one you can use. Or get a hand to.
Those are the main reasons, but there are many lesser ones.
I don't see why carrying another isn't worth the effort.