Did you ever wonder what Han Solo was like when he was eighteen? Ever wonder what V.I. Warshawski's first marriage was like? Did you ever wonder what "Dirty" Harry Callahan was like, fresh from the academy?
Apart from writing a ton of fan-fiction (Which we won't, because we'd rather write stuff we can sell, right? Right? Oh, who am I kidding.) or the creator deciding to write an origin story, all we can do is wonder.
But we can answer those questions for the characters we create. Writing an origin story can be a way to talk to our subconscious (or Muse). Some authors do this whenever they start a new novel or introduce a new character.