What does wearing the Freddy Fazbear Mask do?

The mask prevents recognition, as you sometimes see an animatronic move dangerously close to you, linger, then leave.

Two animatronics aren't fooled, though. These are the Marionette and Foxy.
The Marionette stays asleep as long as the music box is wound (which you can't do with the mask on).
Foxy comes down the the front hallway, and you must repeatedly flash your flashlight at him (which also can't be done with the mask on).

From what I've seen so far, the mask is supposed to be your answer to doors from FNAF. Once one of them gets close enough and is about to come into your room, putting it on will make them walk away once they do get inside.

The downsides to this? You HAVE to keep the music box charged. And you can't do that while you have the mask on. You have to keep going back to the video screen and wind up the music box. If you don't, bad things will happen (because spoiling it isn't as much fun). Also, while you have the mask on, you can't see anything, since you can't use your flashlight and you can't move your head very much to see into the vents.

The Freddy Fazbear mask makes all of the animatronics not reconize you and thinks you are Freddy Fazbear. It only doesn't work on the Marionette ( The puppet ) which technically kind of made the animatronics, as in put the souls into them. Or dismantled Foxy. It's facial scanners are broken so it can see through your disguise.