The answer is: to inform the nation and the world that the US is ready for war if necessary
In Kennedy's speech, the world was full of tension as many saw great possibilities of a new global conflict in the highest point of the cold war. The US president delivers a speech requesting the immediate removal of nuclear missiles from the Cuban island. The speech also is a demonstration to prevent further intromission from socialist Russia into the American continent. The missile crisis, as it is known becomes the mainframe where Kennedy informs the nation the decision and readiness to engage in open conflict.
Important is to know as well that USSR demanded the US government to remove the missiles that were set in Turkey, that also pointed in case of a nuclear conflict.
