In TRTF Classic
The Monitor is a vital game mechanic in The Return to Freddy's that allows full access to video camera footage within Freddy Fazbear's Pizza (excluding the Kitchen, which is audio only) and is one of the many mechanics that consumes power.
In TRTF 2
The Monitor returns in The Return to Freddy's 3. It is very important in game, as on it you can seal 4 different doors, those being in CAM 01, CAM 02, CAM 05, and CAM 11, these being the only defense against Lockjaw, Freddy Fazbear, Bonnie, and Kitty Fazcat.
The Monitor makes its 4th appearance in The Return to Freddy's 4. However, it has almost no use other then to simply view the animatronics. It is possible to beat a night, and by extenstion, the whole game, without using it.