One one level, they foreshadow, even influence the events that take place in the play. They plant the seed in Macbeth's mind, and that of the audience, that the title character will be " thane of Cawdor and King hereafter."  Macbeth ultimately decides to act on the prophecy, and the audience can see this playing out. On another level, there is some doubt left in the audience's minds that the witches were simply apparitions, justifications, perhaps, of what Macbeth and his wife have hoped to do all along. Finally, the witches' prophecy highlights one of the fundamental questions in this play. Is it Macbeth's agency that leads to all the evil deeds in the play, or is Macbeth an instrument of fate, or the supernatural? The witches, by predicting events long before they happen, bring this question into stark relief early in the play. It should also be noted, though it is difficult to know how true this would have been of sophisticated London audiences, that belief in witchcraft was quite common in early modern Europe, and that Macbeth was performed in England as a witchhunt craze swept continental Europe in the seventeenth century. To put it mildly, the idea that malevolent forces could control human events would not have been implausible in Shakespeare's world.