just found out what the keys are used for in the Zombie map Die Rise. Lots of people when the day the new map came out thought it was for special easter eggs and keys to hidden doors, however they are in fact used to call the elevator to whatever level you are on, and also bringing with it any perks that are in that particular one.
Each key can only be used once so its best saving it incase of a player gets downed and needs an important perk back like Juggernaut or if they haven't got the perk yet but find themselves stuck in a difficult spot where the extra abilities may come in handy.
Don't forget that you can jump onto the roof of the lifts and hitch a ride to the next level or up to the roof. Here is some images of the keys being user and how to use them:
When you spawn at start of the level, go down the elevator that crashes to pick up 2 keys in the corridor below: