I always felt that the Batwings for the helmets were originally like the Horsehair crests or specialist helmets members of other Legions wore. To denote ranks like Captain, Sergeant, or even a Veteran, as well as being possible rewards for some Astartes. It's how I've used them in PH Night Lords. Captain/Centurion. Moritat WIPs. Destroyer Squads (WIP).