It is basically a way to extend the content by limiting progression until you can grind / farm the necessary items. Essentially normal gear checks and item progression with a different mechanic.
Instead of giving the player several ways to overcome a challenge, there is just one way; acquire Agony resist. A little bit of a cheap solution imo. Other MMO’s have had a similar systems, LOTRO with “Dread” and “Radiance”, Warhammer with Wards, WoW with resistance as mentioned previously.