Why Do We Need A Ghost?

Obviously a ghost makes our protagonist more interesting, especially if we only learn about it as the story unfolds. Much more important is the ghost's influence on the character's behavior. His or her trauma is the source of the protagonist’s psychological and moral weakness, his internal opponent. John Truby who devotes many pages in his book, The Anatomy Of Story, to that topic, describes the ghost as the great fear that is holding the hero back from action. That's why he or she has to overcome it.