Through the witches only appear briefly in Macbeth, the witches have an impoartant function in Macbeth. The play opens with their grim and stormy meeting, and this contributes to its pervasive tone of mysterious evil. [ Macbeth is a play filled with evil. It lurks around in every scene. Some aspect of it is shown here and there which is why the tone of evil is pervasive.] The witches also serve as an enactment of the irrational. The supernatural world is terrifying because it is beyond human control, and in the play it is symbolic of the unpredictable force of human motivation. [ In other words, humans sometimes act for unpredictable reasons. Not everything is rational and in our control. Weather is one thing. But even human nature, our desires, and what triggers us to do things, such as emotions is not so predictable.] They are the embodiment of the potential evil in humankind.
The play stats with the Witches, stating an ambiguous rule "Fair is foul, and foul is fair." (1.1.11). Their deceptive readings of the future encourage in MAcbeth and Lady Macbeth a false sense of what is desirable or even possible. The magic of the Witches is thus an image of human moral disruption. Through their own uncertain nature, they demonstrate and promote the disruption in the world of the play. When Macbeth meets them a second time, we see that their world is without definition; similarly, MAcbeth's disordered sense of the world comes to encompass the assumption that "Life's. a tale/ Told by an idiot, full of sound and fury,/ Signifying nothing" (5.5.24-28).
Whether or not we believe in the supernatural does not matter. The witches are used as a source of temptation for Macbeth. The witches could very well be a part of Macbeth's imagination or psyche. Whether or not they do exist does not really matter. They are simply used to trigger the ambition in Macbeth.
