Dogs?

One thing that you'll see included in much of post-apocalyptic media is man's best friend.  Max had one in Road Warrior.  Obviously the movie A Boy and His Dog had one.  Even Will Smith had one in I Am Legend.  Why are canines so important?

Well, for one thing, dogs are notoriously loyal.  In most PA settings, there is nothing to enforce laws, and...well the general premise of "dog eat dog" (sorry, unavoidable pun) is one of the biggest rules of survival.  What better companion can you find than one that will never stab you in the back?  All that dog needs is a bit of food, water, and attention and you've got yourself an irreplaceable friend.

Let's not forget the other inherent strength of the dog - his nose.  The apocalypse usually leaves food extremely scarce, and the dog's ability to sniff out hidden goods or even track live prey is an invaluable asset for survival.

In more abstract terms, the dog can commonly serve a very important purpose of toning down the bleakness of the setting.  There isn't much happiness to be found in a world ravaged by raiders, war, and famine.  But unless you're Michael Vick, it's hard not to smile when you see a dog.  It also gives the author or director a huge opportunity to tug those heart strings in the opposite direction in an "Old Yeller" type scenario.  Seriously, even the thought of a dog dying can make people cry.