Mythic+ Dungeon Loot Chest is to farm lower level mythic dungeons ceaselessly in the circumstances that your character in high ilvl. As long as someone offers the Keystone, you can continue to do the Mythic+ Dungeons limitless, to get high ilvl gears and Artifact power, also have a chance to gain Legendary items and some WOW Gold. 

Normally you can complete low-level mythic dungeons in 60% of the set time to reward three chests. The reward chest you got from the mythic dungeon is determined by the completing time: you will reward three chests when finishing dungeon within 60% of the set time, two chests within 80% set time, and one chest when just finished the mythic dungeon or overtime done. The keystone level determined the chest ilvl you will get, higher level keystone you use, better gears you got from reward chest. There is a good opportunity to pick ilvl845+ gears and Artifact power from the Challenger's Cache. Also,  a good bonus getting much artifact power according to your artifact knowledge level, for example, you can gain more than 10,000 artifact power once your artifact knowledge level gets to 15. Besides gears from the chest, farming mythic+ dungeon is also the quickest way to gain legendary gears.
Mythic+ Dungeon Loot Chest will be the best chance to acquire Artifact power and Legendary gears.

At the end of a successful run, a chest (or several chests) will appear that all players can loot. Dungeon Chest drops are taken from the loot table for the dungeon the players have completed.

Class Hall Chest drops may be from any dungeon, including dungeons you have not yet unlocked (e.g. Court of Stars/The Arcway) or dungeons that do not have a M+ mode (e.g. Violet Hold).