The witches, or weird sisters, play a pivotal role in Macbeth as they serve to promote Macbeth's own descent into madness and his unsatisifed "vaulting ambition."
Hectate, the head witch is perterbed that the three weird sisters went ahead "to trade and traffic with Macbeth"(III.v.3) without involving her. She is ambitious and anxious to show "the glory of our art."(9)Her ambition is clear as she tells the sisters that they have gone to all this trouble for one who "loves for his own ends, not for you"(13) suggesting that it will do more for Macbeth's ambition than their own.
To be sure that the witches receive the recognition they deserve, Hectate will ensure Macbeth's fall "unto a dismal and a fatal end."(21)
Later, the witches prepare their potion of "pow'rful trouble" (IV.i.18) as the "fire burn and cauldron bubble"(20). Hectate commends them on their efforts-"And everyone shall share i' th'gains"(40). The witches are aware of their hold over Macbeth and see it as an opportunity to further their own ends. They enjoy seeing Macbeth become more and more frantic to hear new of his future and are happy to "show the best of our delights." (128)
The "foul and fair" mantra of the witches propels the plot forward and cements Macbeth's fate.