It can be surprisingly tough to set down a clear-cut statement of your purpose. In my public-speaking classes I always ask people to state their purpose; usually the speaker is not too clear on this, even after giving a speech. That doubt can stem from an attempt to convey too much, to make sure an audience gets all the facts. Don't be an overloaded speaker who drifts and, at the end, can't get back to your original message. Your purpose should be so clear that no one is left in doubt. When your purpose is defined at the outset, you can make sure all that follows supports your aims and that no one ends up wondering what point you were trying to get across.