What was the purpose of the opening scene in act 4 scene1,prior to Macbeth's entrance

As the act opens, the witches carry on the theme of doubling and equivocation that threads throughout the play. As they throw ingredients into their cauldron, they chant "double, double, toil and trouble"—a reminder that their speech is full of double meanings, paradox, and equivocation (IV i 10). The apparitions that the witches summon give equivocal messages to Macbeth, and they appear to know quite consciously that he will only understand one half of their words. Although Macbeth himself has previously acknowledged that "stones have been known to move and trees to speak" (III iv 122), the apparitions give Macbeth a false sense of security. He takes the apparitions' words at face value, forgetting to examine how their predictions could potentially come true.