NOTE: When you have the right to do something, you don't need a reason to exercise that right; you simply choose to exercise that right or not. But for everyone else who needs it spelled out... here goes.
The same reason the police carry guns. To protect themselves and or others. Do you think only the police ever have an encounter with "the bad guys" during the course of a regular day? How do you think all the murderers and rapists got into prison? Did all of them attack only police officers or did they attack mostly regular people?
The security of a free state. Whether there are terrorists or enemy soldiers attacking American soil, or just a massive breakdown in civil order and the National Guard is 12 hours away from trying to protect your neighborhood after all the police abandoned you, you need to have an asault style rifle to defend your home and family against multiple threats at hte same time.
Banks, Liquor Stores, 7-11s, ATM Machines, secluded parks/Forests and Parking Lots/Garages are all very typical places where the criminal element can strike. Don't forget about car jackings and home invasions either. These are all places where you will need a gun for self defense.
Farm/Ranch. City folks are absolutely clueless how far out in the wilderness their food comes from and that there are actually people who live out there. Wild animals are a constant reality and a 911 call will bring out the nearest Sherriff deputy in about 45 minutes or more. When you live that far out in the middle of nowhere, it is likely that you will need to go hunting for your own food and defend yourself against wild animals using one or more firearms.
Interstate Traveling. Enough said.
Official Answer: When is it that you don't need a gun?
