Very practical skill period. You never know when your dominant hand may be injured or perhaps damaged in a struggle with assailant your trying to defend against. It could be shot, cut, you could fall over and break/dislocate a finger or thumb or any number of unforseen events in a situation that is not going to pause and allow you to learn to use your other hand.
Heck I even would like to practice aiming instinctively with both eyes for the same purpose. Now that is tough. Trying instantly lining up the sights as you bring the gun up with your non dominant eye without closing the dominant one. You never know when one of your eyes could have something in it or be injured, maybe you have a chemical or pepperspray in them and one is more clear than the other.
Practicing to use whatever will still be available assuming your still capable of defending yourself is a good skill imho.
I have even practiced one handed with both hands using a long gun to quickly load, and aim, and with a pumpgun to load rack and shoot one handed. I have shot them one handed accurately at the range as well. In fact for a long gun I find the quickest way to bring one with a shoulder stock to bear one handed is to jam the shoulder stock under your arm and clamp down on it with your arm/armpit as you bring the barrel up and fire holding it between your arm and your body. Even with a 12 gauge and magnum shells I managed shooting it that way easily several times with nothing but a little redness to show for it(be sure to grip it very tightly to compensate for no forward support so the barrel does not fly up)
Handguns are easier, but practice cocking and loading one handed, though I wouldn't practice cocking a loaded one one handed when you can use an unloaded one.