Zombie Apocalypse, Alien Invasion, Red Hordes, Obama Takin' Mah Guns Away, Meteor Impact, The Rapture, UN New World Order Coup Attempt, We Have To Fight The Terrorists Here Because We Didn't Fight Them Over There, America Collapses Because We Let in Too Many Immigrants.

It's basically a silly paranoid fantasy that gun owners have about what they'll do when social order breaks down, either locally or nationally, and all the Rugged Individualists have to fend for themselves in the resulting anarchy, hunting for food and fighting off roving bands of Mad Max-style raiders (or just black ppl and/or Mexicans). The worst offenders are those with tricked-out AR-15s (again, no offense Anony) and 10,000 rounds of ammunition in their closet.

That said, if you live in a potential disaster area (hurricane-prone area, maybe near a nuclear power plant, something like that) and believe in being super-prepared, it might not be unreasonable to have a decent weapon (a rifle is best) that can be used for self-defense in the event that there is some kind of disaster and the authorities don't respond adequately, because it is conceivable that there could be looters or even violent criminals taking advantage of a situation like that. The fact is that it did happen in New Orleans. Of course, gun owners who don't live in such areas are just as enthusiastic about "SHTF" rifles as those who do. And of course gun owners who do live in such areas often put a lot more thought/effort into their SHTF rifles than they put into other, more realistic, survival/emergency-preparedness measures.