Character building
As a reader, there was nothing I hated more in a book than having a main character vomit their entire life history in an attempt to explain some previous reaction or inaction. It's a classic case of violating the SHOW don't TELL mantra that writers all chant to themselves in their darkened basements. It's also just sucky. As a writer, it's important that your characters have some sort of history to make them three dimensional. It informs how the characters would react in a given situation and can allow you to leave a trail of breadcrumbs to entice the reader to follow. Take a classic example from the movies: Darth Vader. Using a nice little bit of misdirection with Obi-Wan Kenobi - telling Luke his father died - the reveal in The Empire Strikes Back that Darth Vader was in fact Luke's father caused the world to choke on its popcorn. From recent internationally best selling books you have the character of Severus Snape. Harry Potter spends almost the entire run...