Why does Barack Obama allude to famous statements from other documents in his speeches?

It is a longstanding custom for professional speakers to use allusions (references) to other documents or books or articles when giving a speech; such allusions can give a speech more authority or make it more interesting to the audience. Political figures will do this to show that their ideas have been supported by others in the past, or they will do this to show that certain ideas and beliefs were true before and are still true today. That is undoubtedly while President Obama uses allusions and references in some of his speeches.