Who needs to see a character’s back story? Not the reader, usually, but it is essential that the writer knows the back stories of principal characters, at least. And they must be well written. If the writer can’t take the characters that seriously, how can the reader? Besides, there might be another useful by-product: an illustrative scene from a character’s back story can easily lead to an excellent ‘slice of life’ short story – one which is capable of standing on its own. I have found some of my best stories by this route.