A brief character background with the broad strokes of what happened to a character prior to them showing up on the stage of the campain can add to the enjoyment of the game for both player and game master. It provides some context.  It can help the player immerse themselves into the role of their character. It gives the game master and other players a tool to play off of during the game. It can be a call back joke that lasts as long as the character does, maybe longer.

If you write a 12,000 word character sketch like a novelist would construct, then what you have done is told me who your character is. I would very much prefer that you show me who your character is by their actions during the play of the game.