The witches appear in the play as three mysterious and supernatural figures who use their powers to influence the course of events. They make a series of prophecies to Macbeth, which set him on a path to destruction. By introducing these supernatural elements into the play, Shakespeare is able to create a sense of uncertainty and unease in the audience, as they are left wondering what the witches' true motives are and how they will ultimately affect the characters.
The witches' prophecies are the catalyst for Macbeth's ambitious drive for power. His desire to become king leads him to commit a series of murders, which eventually consumes him and results in his downfall. By using the witches as a means of exploring Macbeth's tragic trajectory, Shakespeare is able to show the destructive power of ambition and the dangers of giving in to temptation.