If Macbeth had never met the three witches, the events of the play would probably have taken a different path.
When Macbeth and Banquo run into the three witches, Macbeth is greeted by them by being called Thane of Glamis, Thane of Cawdor and, finally, King. Macbeth is, indeed, the Thane of Glanis, but he is confused by the other titles the witches call him by. Immediately after the witches disappear, Ross and Angus arrive to tell Macbeth he is now the Thane of Cawdor as well.
That prediction coming true awakens Macbeth's ambition. He, influenced also by his own wife, will from then on do terrible things in order to become king. Had he not met with the witches, that ambition might have never crossed his mind nor guided his actions.