Yet another thing present in the Division for quite a while now. It's amazing that this is still missing in D1, let alone D2. For the uninitiated, these sets gives you a huge advantage with certain skills. Imagine getting a % extra damage for every single bullet hit from a fast firing auto rifle, and also a damage reduction if you don't miss. Another set gives you a 50 percent chance to get all your life back when reaching low health, and then goes into cooldown. These might have changed, but back then it gave the player that extra push they needed for the really difficult missions (legendary). That might be why Bungie let this one slip, since everything here is "just slightly more" or "just a little bit more".