The Featured Image Property of a Post defines what graphic will appear above the Title of the Post. ALWAYS save Posts with a Title, even if the Featured Image includes the Title you want to use, like an Event poster. The Title is part of the name of the Post and it aids in Searching.
This arrangement of Image over Title can be useful when the photograph of a person is the Featured Image, as in a book review. Then you could start the post with a small graphic of the book so folks know what it looks like.