Ive beaten the game (no spoilers) and I still haven't found what they do. Of course I haven't collected them all yet. Has anybody collected them all yet?
Bring back the modern day or die
Maybe they unlock something in modern day?
In the guide book it says that they unlock cheats for the game. I'm guessing similar to the animus hacks from AC III.
But the animus fragments are part of the challenges, so you can't unlock all the cheats without them.