It serves as pretty much the only kind of restriction that ensures, that not everyone can queue into it and ruin the matches of their teammates. It is endgame PvP after all. Id prefer a requirement that is actually about PvP instead, but I guess that will never happen. Of course this wouldnt be necessary, if they just used a ranked based competitive mode for an endgame mode, but I guess that is one of those things were uniqueness takes priority. Competitive should have been what Bungie made trials.