The companion App is pretty awesome.
I dont think there is a point, per se, to the co-op like we experience in most games.
The Story is pretty weak overall.
HOWEVER!
The game, I think, knows this, and didn't try to add anything to it right now. The focus of the game is to run around like a monkey and fight zombies in a Post-Apoc setting before the Governement comes in and Nukes you.
I think it works because its not serious, and it doesn't attempt to have a story really. (Its much like Minecraft in that regard)