Just for the hell of it. It existed as nothing but a free-for-all stage, which you play for a bit of amusement before you go "ok, I'm done." But after taking it to this extreme, I don't think I'll ever be able to touch the game again. Which is good, I guess, since I need to stop doing that every couple of months.
Fun fact: Zombie hat trends change every 20 days. My favorite is the cowboy hat trend. Although backwards baseball cap trend is quite amusing, too.
Not so fun fact: I can confirm that those bastards do in fact get more ridiculous with their zerg rush habits as days progress, and it's more than just what city you're in.
Fun fact: Going all the way back to your little tool shop bunker at the start of the game with 4 other dudes with automatic weapons and a rocket launcher in your hands is hilarious. Sure, they're meaner when you get back there, but they can still only pour in through that tiny door.