My short answer to a recent writing site prompt asking, “What is the purpose of backstory.”
For me, there’s two flavors of backstory…
If the backstory is part of the story, then it is where the hero’s Problem lurks, the one they trip over in the inciting incident and must battle throughout the story, until they finally see it and stomp on it in the end. Although stories seem to be about a battle with the Antagonist, in the end it is usually winning the battle with some part of themselves that allows them to win against the bad guy.
If the backstory is not shown to the reader, then it’s often used as a bible of sorts for the writer to follow to inform who their character is as a three-dimensional, fallible–and often broken–person. It helps them know what their character will do and feel in any situation the writer throws them into.
It pays to get to know your characters from beginning to end. The more “alive” they are, the more they will bring your stories to life, too.