Speaking from experience with Destiny, its mostly to define end-game content. While scaling is applied to almost everything, they usually tick on the extra levels for the newest thing so that you go into it with a bit of a disadvantage and added difficulty (at least initially.). Why they keep hopping back and forth between regular leveling and gear-based levelling is beyond me (Particularly since infusion lets you use any gear after the year 2 switch anyways. Its not like Vanilla/TDB where you had to run the raid/Iron Banner armor for the cap).
In what is basically one of the games greater flaws, merging skill-based (FPS shooting) PvP with the PvE elements instead of running separate loot/balance kind of leaves most of the loot lacking. None of the guns get to do anything too unique, because it would create an imbalance (since only certain persons would have rarer loot items) in their PvP playing field. Thus, the only way to power up the guns (or depower them to create challenges) is to fiddle with the numbers.