What does the music box do in fnaf 2

The music box's in-game purpose is to keep 'The Puppet' pacified. The Puppet is one of the hostile animatronics, and while the music box is playing, you are 100% safe from it. However should the music end, the Puppet will start to emerge from its box. It is possible to keep him there, but should he leave it fully then death is almost certain, it can be as little as six seconds or as much as two minutes, but the Puppet will approach and kill you with no way to prevent it. Your only hope is for it to hit 6am before the attack. So you must keep winding the box, which means during that time you can't see what the other animatronics are doing, so it's up to you whether you risk winding it fully, fewer times, or winding it for less, but more often.

In terms of game mechanics, the Music Box is the reason why a player cannot just permanently wear the Freddy Fazbear mask and wait the night out, as none of the other animatronics would attack him if he did so. You cannot view the monitor while wearing it, and therefore cannot keep the box wound. Unlike other animatronics who ignore you while you wear the head (unless they 'see' you first and you are too slow in putting it on) the Puppet is not fooled and will kill you anyway.