Why do you carry a BUG on duty?Ankle guns. When I'm on duty I carry one. But off duty I dont.
Then ask yourself, why isn't that reason valid when I'm off duty? After all, you carry your cuffs and badge, and said that you would intervene if necessary. So if you are willing to take the same action on duty or off, shouldn't you have the same preparation?
As to why carry a second gun? Because guns are mechanical devices that can break. I bought a new gun and took it to the range to practice. I ran the popular "200 round reliability test" to determine if it would be OK to carry. The extractor broke on round 201 and the gun was down for the count. If that had been on the street and I didn't have a backup, where would that leave me.