For example, Shakespeare uses the metaphor of darkness to illustrate Macbeth's slide towards depravity. He cannot face his deeds but needs the dark to hide what he is doing:
Photographers and artists are always looking at light as the key element in any composition, which is certainly extremely important for dark advertisements.
Dark ads are usually used to portray a scary situation, or a serious issue, creating a dramatic impact. Light represents a way to overcome problems and solves negative situations.