This is a much more interesting question than the o/p perhaps understood.
There is legitimate scholarly opinion to the effect that Shakespeare did not include any Witches in his play. The modern notion of 'witches' is a misunderstanding; or perhaps may be classed rather as an exaggeration.
The text which modern academics ascribe to Shakespeare speaks of 'weird sisters', not of witches. There is much ambiguity in this play, as in many of his plays, due to corruption of the text during the process of printing it, under the exceedingly primitive conditions so long ago. The printed text contains much textual corruption.
But, above all, it is now thought that some scenes which appear in modern editions of the play were added by another author, perhaps in the years between Shakespeare's death and the First Folio printing. The references to witches are thought to have been added by another hand.
Calling them witches certainly attributes supernatural powers to them, such as could account for their having an ability, with some credibility, to foretell the future. But although Shakespeare is still thought to have written the scene in which Lady Macbeth appears to urge her husband to murder King Duncan, some part of the ambiguity in her words can now be accounted for by the fact that she may be referring to a rather different version of Act 1 Scene 3 than is currently included in modern printings of this text. Shakespeare may have written her dialogue without any witches in mind, and they may have been added by another author so as to strengthen Lady Macbeth's position in this scene, among others, where she needs to call on a supernatural prophecy that Macbeth 'wilt be King hereafter'.
In the 16th century, playhouse audiences loved magic -- which most common people still firmly believed in. So adding witches to this play certainly strengthened its appeal for audiences. By adding supernatural elements, Shakespeare had improved other plays -- such as where he uses the ghost of Hamlet's father to include a ghost in that play.
Yet he seems to have only had three 'weird sisters' in mind in Macbeth. Someone unknown made the play a better box office attraction, by inserting just one extra scene with a reference to witches -- throughout the rest of the play the term was not originally used -- to give a better justification for Lady Macbeth's claims that fate has prophesied that Macbeth will become king.
Her words, perhaps made ambiguous by simple textual corruption, but maybe because they originally referred to a quite different scene to the one now included in the play, have very little meaning. But the reason seems not to lie with Shakespeare but with the unknown hand who "collaborated" with him on the script - perhaps long after his death.
Indeed the Folio text of 1623 is thought to be a revision of the original play, probably adapted by Thomas Middleton (and unquestionably using Middleton’s material), and is very short by Shakespeare’s standards, suggesting abridgement.