The ambiguous words have the effect of encouraging Macbeth to continue pursuing his tyrannical measures and to maintain his confidence and courage to the very last. They give him the will to fight even when his thanes are all deserting him and an overwhelming English army is approaching. This makes the finale more colorful, dramatic, and theatrically impressive. The alternative would have been for Macbeth to try to flee the country in disguise. He has committed so many atrocities that he cannot hope for any truce or leniency. (He is a lot like Hitler in his bunker with the whole Russian army approaching, determined to hold out to the very last.) The ambiguities also finally have the effect of suggesting that Macbeth's fate was inevitable and inescapable from the very beginning. The witches have been telling him the truth all along, but he has chosen to interpret it in the sense most favorable to himself. (This is human nature.) He was destined to be king and destined to "be bloody, bold, and resolute," a real tyrant, until the very end. In his confrontation with Macduff in Act 5, Scene 8, he realizes that he has been tricked. He says:
And be these juggling fiends no more believed
That palter with us in a double sense,
That keep the word of promise to our ear
And break it to our hope.