Using the witches to give a prophecy would have greatly appealed to an Elizabethan audience that very much believed in fate and those elements of the supernatural. Just like Macbeth was inclined to believe the witches' statements, so, too, would have Shakespeare's audience.
In Act IV, Scene i, the witches summon apparitions to answer Macbeth's questions. A floating head first warns him to beware of Macduff. A bloody child tells him that no man born of woman will ever hurt him. Third, a crowned child tells him he will be king until Birnam Wood moves to Dunsinane Hill. Then the witches vanish.The appearances of the apparitions continue the dark mood and the mysterious supernatural forces that have been present throughout the play.
In Act III, Scene 5, Hecate is angry that she was left out of meetings with Macbeth and tells the witches that Macbeth will see apparitions that will, "by the strength of their illusion" lead him to believe no harm can come to him. At the end, she says, "And you all know, security/Is mortals' chiefest enemy." Making Macbeth feel he's invincible will lead to his downfall, according to Hecate. In other words, the images of the apparitions will be so strong that they will add believability to what they tell Macbeth. The "illusion" in this case is meant to mislead and deceive Macbeth into feelings of security, a man's "chiefest enemy". Macbeth won't question what they tell him because of the way they look, and they tell him what he wants to hear. Just think of the impact a floating head, a bloody child, and a child wearing a crown makes on Macbeth! Using the apparitions is another trick of the witches to fool Macbeth into a false sense of security.