In the witches' second scene, moments before Macbeth's first entrance, they say "Peace! The charm's wound up", implying that they have cast some sort of spell.
So before we even meet Macbeth, there's a suggestion that he might partly be under the witches' control, or that there might be a spell cast on him. And we never know whether that's true or not.
The witches then make predictions about Macbeth become thane of Cawdor, thane of Glamis, and King. Macbeth then kills the king, and everything happens as they predicted. But would it have happened if he'd just left well alone? Is Macbeth's own suggestion, below, true:
If chance will have me king, why, chance may crown me,
Without my stir.
Is King Macbeth the result of a spell, or of Macbeth's actions? Where does the supernatural influence start and finish? Would it have happened without the murder of Duncan? It's unlikely - but it's impossible to tell. The same problem applies to their later prophecies in the apparition scene.
Then, to add to this conundrum, you have the status of the "air-drawn dagger" which leads Macbeth to Duncan, and the appearance of Banquo's ghost. Are these made by the witches, or imagined by Macbeth:
....or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
The play is full of supernatural solicitings. But their significance? That's open to interpretation.