While there have been extensive discussions of the Elizabethan preoccupation with the supernatural and Shakespeare's use of this element to appeal to his audiences (refer to other questions and answers in this Macbeth group), the witches do contribute to the tragedy. For, they provide the impetus to Macbeth's tragic flaw of his unreasoning drive for power. With the predictions of the "weird sisters" Macbeth rationalizes,
If chance will have me King, why,/chance may crown me,/Without my stir (I,iii,144-146).
As the witches all chime together,
Fair is foul, and foul is fair./Hover through the fog and filthy air (I,ii,10-11),
Macbeth readily enters this fog of superstition and magic as his means to rise meteorically to power. With his acceptance of the preternatural order of events, Macbeth can commit his heinous acts more readily and without compunction.
And, to further the tragedy, his wife is snared into the preternatural world as she unsexes herself to aid in Macbeth's heinous acts:
Come, you spirits/That tend on mortal thoughts, unsex me here,?And fill me, from the crown to the toe, top-full/Of direst cruelty!Make thick my blood,/...And take my milk for gall, you mrd'ring ministers,...(I,v,40-48)
Tragically it is not until she commits suicide that Lady Macbeth returns to her human and feminine self, proving again that it is a fateful power that the evil witches hold in "Macbeth," a power that evokes the tragic flaws of both Macbeth and Lady Macbeth.