I think it’s helpful for characters to have certain mysteries that are never explained. After all, we never know the whole backstory (even the important parts) of people we know intimately. Knowing the character’s backstory can help an author decide what details to leave out in order to create mystery.

But although knowing a character’s entire backstory can be helpful for the author, even if the history is never completely spelled out in the narrative, I think this kind of pseudo-knowledge can lead writers into thinking they know everything there is to know about the universe they’re creating, when their writing may be more interesting if characters are able to hide something from the author.