Were you looking for the Camera Flash from the second game?
Normally the Bedroom is sourounded by darkness. There's a small circle around player's mouse that partially brightens the room, this is the flashlight. The flashlight is a really vital mechanic, as it makes the player able to spot any threats around the room.
Another purpose of the flashlight is to scare away Monster Rat. Once the player hovers over Monster Rat's eyes, thus flashing the light on him when he is at the bedroom's door, wardrobe or closet, it will repel him for some time. Once he is in the room, player must do the same to avoid getting killed. However Monster Rat will try to dodge the flashlight in this situation.
Finally it's also used to make him leave the room. After the previous situation mentioned above, Monster Rat will always go underneath the bed. The player must then look under it, and figure out under which side of the bed he is hiding. Then leave and while looking on the room, flash the light on the opposite side of the room. It will make so that monster will peek out his head on the other side, which is when the player must flash the light onto his eyes. Performing this operation correctly will make him leave the room.
Remember, you can only see Bedroom brightened in a small radius around your mouse. You can't brighten the whole room, so it's different from using the Camera Flash in the second game.