The most exciting & detailed-out supernatural element in Macbeth is the Witches. The three witches & their queen, Hecate, not only contribute to the dark, uncanny atmosphere, they also highlight the problematic of evil--the temptation & the fall. They may resemble the 'fate sisters' in Classical mythology as well as an ironic reminder of the temptation of Christ by three tempters. Macbeth, the admired saviour of Scotland, is Christ gone wrong. By making their proclamations on the heath, they exteriorise the germ of evil in Macbeth. Apart from remarkably adding up to the visual theatre of Macbeth, the witches are responsible for Macbeth's doing; later in the play, the three apparitions conjured by the witches equivocate to his undoing.
The air-drawn dagger seen by Macbeth just before the murder of Duncan, and the ghost of Banquo, appearing at the banquet are two more supernatural elements which illustrate the subjective & psychological version when intense fear & guilt assume the form of supernatural visions.
In Macbeth, Shakespeare uses the supernatural in all possible dimensions--theatrical & psychological, objective & subjective, realistic & symbolic.
In Macbeth, as in Hamlet there is a distant element of the supernatural which lends it a flavour of its own- a mystery and awe; a wierd enchantment of atmosphere which forms it instant appeal.
The wierd sisters may be anything except fates.They have no direct share in the action and yet form an integral part of the play.They and thier prophecies represent not only the evil tendencies in the hero's soul but also the unknown vague influences of evil around him in the world which help his own ambition and the incitements of his wife.
The witch-scenes deepens the sense of fear, horror and mystery which pervades the atmosphere of the tragedy.Without its peculiar atmosphere nothing remains of the tragedy and without the witches the atmosphere is incomplete.