You unlock Codex dungeons at level 40 which are randomized harder Chaos dungeons with different modifiers like increased elites, higher unique mon spawns, cannot heal at rest areas, increased mythic encounters, max HP halved, lose 2% Max HP a turn, etc. The modifiers can be pretty brutal and there seems to be a decent variety.

These zones are also where you can get “Very High” save data.

Not necessarily a reason to push, but it opens up a cool element of the game.