Mythic+ is where gear and rating come from in Midnight, and it is the grind people quit. This shelf covers every level of it: Mythic 0 clears for your first pieces of gear, timed key runs at the level you need, a score target if you are chasing a Keystone achievement, and the dungeon portals that save you a flight every week.
Runs go out with a full group of raiders who play these dungeons daily, so keys get timed rather than bricked. Loot traders are an option on the key runs if you want the drops from the group as well as the chest.
Keys, score or achievement?
Buy key runs when you want gear and vault slots. Buy a score boost when you need a specific rating number by a deadline. Buy Keystone Conqueror, Master, Legend or Myth when the reward you actually want is the seasonal achievement and its mount.