its probably just is damn autograph! if its to be used for anything it would be used to prove that luffy is Ace's brother. or ace could be saving a spot for luffy at the next pirate summit and thats proof that luffy is Ace's guest.
or perhaps its the missing piece of the 100 year history gap but luffy is too stupid to realize it
or maybe that tiny peice of paper is actually the One Piece and, yet again, luffy is too stupid to realize it.
or my personal fav, its a list of Ace's favorite cuss words...blahz...