The witches, who are also called "the three sisters," an allusion to the mythological goddesses who controlled the fate of humans, delight in manipulating mortals.
This manipulation of the lives of mortals enhances their sense of power as well as providing them enjoyment. In Act I, Scene 3, for instance, the three witches boast to one another of their accomplishments, their evil and disrupting acts. For instance, because a sailor's wife would not share her chestnuts with the first witch, the evil spirit decides to whip up a terrible storm for the husband who is at sea":
But in a sieve I'll thither sail,
And, like a rat without a tail,
I'll do, I'll do, and I'll do. (3.1.7-10)
Certainly, these evil sisters are an intrinsic part of Shakespeare's play, fueling the character faults of Macbeth and motivating his ambitious actions; for instance, when they call him the Thane of Cawdor and Macbeth is later awarded this title by King Duncan, Macbeth then believes that the witches know the future. This credulity allows the witches much pleasure as they can exercise their many dark machinations.
In addition to their important contributions to the plot of Macbeth, the witches reinforce the cultural beliefs of the Elizabethans who felt that the supernatural played an intrinsic part in their lives.