Prophetic fallacy is used effectively because tragedy and suspense are created when Macbeth does not expect the prophecies to fail.
Prophetic fallacy refers to prophecies turning out different than a person expects, resulting in disaster.
The play begins as three witches inform Macbeth that he is going to be promoted to Thane of Cawdor and then King of Scotland. At first, he laughs off the prophecies. Yet one the first comes true and the second does not, he gets annoyed.
That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires… (Act 1, Scene 4)
Throughout the rest of the play Macbeth acts in firm believe that the prophecy is right. When he gets the second set of prophecies, he feels more confident. The witches basically tell him that he is invincible. It seems that no man can hurt him, and he won’t be in trouble until the forest comes calling.
In the end, the prophecies come true. Macbeth can be hurt, because Macduff was born of C-section. Malcolm tells his soldiers to cut down tree branches to disguise the troops, so the forest does come to Macbeth. He is vanquished, all because he believed the prophecies a little too well and did not read between the lines to see what they could actually mean.