Part of storytelling is creating compelling characters.
And part of creating compelling characters is that you give them an origin story.
An origin story is a story that explains how a person became to be who he or she is.
Let’s look at Spiderman.
Spiderman was a normal teenager named Peter Parks until he got bitten by a radioactive spider and his uncle was murdered. Peter blamed the murder on himself, and this motivated him to fight crime.
But origin stories are not just for superheroes. Origin stories are for every character that you want people to relate to.
Cracked has a funny cartoon on How to Create a Plausible Superhero Origin Story.
Solo Biz Coach also has an article for brands on how to develop an origin story.