The visual image of a speech can be one of a circle. Every speech has an opening, middle and conclusion. Many novice speakers think of a speech in terms of a straight line. They start with the opening, leading into the body and ending with a conclusion. Of course after the conclusion of the finished speech, there appears to be nothing. It is similar walking up to edge of a cliff and seeing nothing but the ground below and the blue skies above. Most travelers when coming to the end of the road are puzzled as to where to go next.
However seasoned speakers see their speech as a circle. The conclusion ties back to the beginning of the speech. If in the conclusion they issue a challenge, they want to refer back to their opening to remind the audience as to what they said at the beginning. Many endings have a call to action or an idea for the audience to think about and review. The opening can be a small step to that end result with the conclusion completing the action.
Think of a car which can’t go forward or backward unless it has very circular wheels.