It also shows that this type of downfall can happen to anyone, even the best of men. It reminds us that it is important not to be overcome by blinding ambition so much so that we stop listening to our conscience or those around us.
Macbeth is a tradgedy. The fact that he was described as a brave honored soldier at first made his eventual downfall, due to his nagging guilt and doubt, all the more tragic. This question was also answered beautifully by ben-alberstadt and fexxiwig at http://www.enotes.com/macbeth/q-and-a/why-important-that-macbeth-should-first-presented-122061. Best of luck!