What is the dramatic purpose of the witches as created by the playwright?

The witches serve as dark forces that attempt to bait the dark forces within Macbeth and the human psyche. They tempt Macbeth with suggestions, "Win us with honest trifles, to betray his purposes" in the end. They show up first because Elizabethans were really scared of witches, they were instruments of darkness.They hooked the audience and kept them glued to the action. Shakespeare uses the witches to effectively convey the motifs of darkness and equivocation throughout the play.