A firearm is somewhat a multi-use item. From the main uses of hunting and self-defense (from animals two legged and four), to possible uses like fire starting, signaling, and whatever else you can think of. Not as multi-use as, say, a knife; but it can do certain things that would be tough to accomplish without it.

Like many said though, what use you have for a firearm depends on what you're planning for and what your environment is like.

If you're only planning for a best case scenario, say spending a day or two in the Pine Barrens until you can make your way to safety (or get rescued), you're probably not going to need a firearm. If you're planning on doing a three week trek through Alaska, or you're planning on making your way out of Newark during a riot (like my grandfather in '67), it becomes a more important piece of kit.

I think the reason most people consider taking a firearm as part of their gear, is that it's very traditional when it comes to outdoor living. Just like no early American settler, trapper, or cowboy would be caught without their knife, their firearm(s) wouldn't be far behind. It was useful then, so it could be just as useful now.