To unlock the secret movie
Once you've beaten Bravely Default, a secret movie will unlock that essentially serves as a bridge to the upcoming sequel -- Bravely Second. But apparently you don't need to complete the game to unlock it, as you can use the Konami code (up, up, down, down, left, right, left, right, B, A) on the main menu to get it.
Be warned -- this movie sort of reveals the fate of one of the characters out of context. I'm not going to spoil the secret movie, but I will say that it got me sufficiently pumped for Bravely Second.