They are also used at the perk sode machine to buy perks for your character.
The guns will come with various upgrades and mods, so be careful when buying these. You can use it to buy new guns that are found on walls. You can then use it for various things in the game. You get it by killing zombies, completing objectives, doing side missions, and finding vials of it in certain rooms.
Zombie Essence is basically your score and your cash.
