How does the witches rhyming couplet refrain add to atmosphere?

The witches' rhyming couplet refrain, "Double, double toil and trouble; Fire burn and cauldron bubble," establishes a supernatural mood and creates a sense of mystery and foreboding. The rhythm and cadence of the refrain give it an incantatory quality that transports the audience into the realm of magic and witchcraft. By repeating this incantation throughout the scene, Shakespeare enhances the feeling of tension and suspense, immersing the audience in the dark and ominous atmosphere of the play.