In the Elizabethan age there was a renewed interest in the supernatural and people blamed unexplainable events such as the Plague upon the work of witches, especially what was termed "black witches." Such "black witches" were the three in "Macbeth." Their purpose is to create evil and manipulate human beings in order to create havoc and harm.
In "Macbeth" Shakespeare employs the witches for several reasons. Opening the play with the thunder, darkness, and three witches stirring a cauldron creates great dramatic effect, attracting the attention of the superstitous audience. In addition, Shakespeare sets the tone for the play by using the light vs. darkness and evil; he also presents the motif of moral choices and religious ideas. The use of the witches is effective with a character of such cupidity as Macbeth who succumbs easily to their seductive evil influence. Unlike Banquo who is cautious in believing the witches
...But 'tis strange:/And oftentimes, to win us to our harm,/The instruments of darkness tell us truths,/Win us with honest trifles, to betray 's/In deepest consequence (I, iii,122-126).
Macbeth succumbs to the charm of political advancement:
If chance will have me King, why,/chance may crown me,/Without my stir (I.iii.144-146).
Another use of the witches and supernatural is in Shakespeare's motif of appearance vs. reality which runs throughout the play. One such example is in the vision of the dagger. Is it a hallucination of Macbeth's or a vision sent by the witches? At any rate, the supernatural controls the character of Macbeth and this is the purpose of the three witches.