I suspect you're having trouble because you haven't clarified in your own mind what you mean by "purpose." Frankly, I don't understand what you mean by purpose. Shakespeare wanted to entertain his audiences and thought that the story of Macbeth would entertain them. He wanted to fill theater seats and make money. He also wanted to be admired for his beautiful language. The audiences came to his theater because they wanted entertainment and perhaps some edification. A prominent writer named Walter Bagehot said that what we all want is escape. We want to be transported away from our daily worries and boring routines.
All the best stories in the world are but one story in reality--the story of escape. It is the only thing which interests us all and at all times, how to escape.
When we read Macbeth or watch a production of Macbeth, we are intrigued, thrilled, frightened, and transported in imagination to a different place and a different time. We think about the story afterward. We have probably learned something through having had a vicarious experience. Shakespeare's "purpose" was to provide an illusion of reality. He wrote in Hamlet: ". . . the purpose of playing, whose end, both at the first and now, was and is, to hold, as 'twere, the mirror up to nature, to show virtue her own feature, scorn her own image, and the very age and of the time his form and pressure." (3.2.19-22) You might want to use that quote in your speech. Good luck!