In the YouTube video posted here, the player went through the main menu screen and there is an "Extras" like Borderlands 2. There is a place to enter shift codes, any idea(s) what the codes will get you? In BL2 you got to open a chest that gave you an item that was at your characters current level.
shift codes= graphic updates, added a.i. line codes, and added campaign content through script changes, and skin changes.
In all seriousness though, they'll most likely be used for double xp, weapon skins(since they had an "abundance" during game creation) as well as a combination of the reserved items(gamestop exclusives) would be my best guess.
I honestly can't wait to see what's in store for this game as I'm having fun with it, and it can only get better from here.