The full theory goes like this:
The previous gaurd (Mike, from FNAF 1) complained about killer robots. He has been moved to the day shift. As a result of his complaints, you have been given a Freddy mask. This mask will make the old animatronics think you're just a suit. The new animatronics were given facial recognition so they won't kill you. They have also been programmed to dismantle the old animatronics. So if they catch you wearing the freddy mask, they will dismantle you. If the old ones find you without the mask, then you will be stuffed in an animatronic suit, just like in FNAF 1. So the point of the game is to wear the mask to protect you, but you can't just wear it all the time.