Answer:
I have been in theater for quite a while, and we always leave one light on. This light is called the “ghost light” and while there is always the argument as to what it’s purpose is, whether it's there to chase away mischievous spirits or lights the way for the ghosts that are said to inhabit virtually every theater and therefore keeping them happy and content, at my school it was seen to keep the spirits away. In reality, the function of the light is to make it so the light cast can always find their way to the control panels, it is surrounded by folklore.