The mask was supplied after complaints were filed "about the characters trying to get into The Office" by the previous guard. It is used to fool any animatronic (excluding Foxy and The Puppet) that enters The Office into believing that the player is an animatronic themselves.
Ultimate Custom Night
The Mask returns in Ultimate Custom Night to fend off Withered Bonnie, Toy Bonnie and Toy Chica while they are in the Office. While Toy Bonnie and Toy Chica can enter at any moment they are easy to fool, when Withered Bonnie attack, the player has a shorter time to put on the mask, however he can only enter when the player is watching the cameras.
The head can also be used to make Golden Freddy disappear, but it can also be done by flipping up the monitor.