Have you noticed that certain grimoire cards can confer in-game bonuses? Case in point: • Enemy cards, such as Dregs, Vandals, etc., confer extra glimmer while a glimmer collection item is active, provided you have killed enough of that enemy type to unlock the bonus node. • Weapon type cards, such as Auto Rifles, Scout Rifles, etc., often provide an experience to weapon type boost when leveling up new weapons, provided you have killed enough enemies with the weapon type to unlock the bonus node. • Crucible playlist cards, such as Control, Clash, etc., often unlock class items in your loot table while playing said playlists, provided you have won sufficiently many matches in said playlists for the respective bonus nodes.

bro, they are grimoire, the extended story of the Destiny universe... they lead to future events and tells us more about the game [spoiler]Prince Ulderin is alive, confirmed by ghost[/spoiler]