I'm almost Rank 6 with Dead Orbit. Every rank you gain past Rank 3, that Faction sends you a Legendary gift through the Postmaster. I've received the Dead Orbit Fusion Rifle, Oblivion Gauntlets, and Rocket Launcher from ranking up in the past (hoping for their Auto Rifle since i accidentally destroyed the one I bought...), but the package could also be Legendary upgrade materials, Motes of Light, and Strange Coins. It's like the RNG, but with a smaller pool of options because any weapons and equipment are from that Faction (which is cool in itself). That, and I have a sneaking suspicion that they're going to include stuff in later updates and/or expansions that deal with the Factions and having higher and higher Rank. I have nothing concrete to go off of, just a feeling I have.