Basically just a way to get new life out of old dungeon maps.
You cue WITH your chocobo, and the mobs are similar but different. The focus becomes more on exploration... perhaps you find pieces of choco barding/rider's attire, that you can take to an NPC to repair and make whole.
For example... traveling through Amdapor Keep, you'll have different areas that will let you click and hear of the history of the place. Maybe an NPC follows you and talks story with you, but random bits of the story unfold each time you go in.
For instance, perhaps a statue you stop at and the NPC tells you about the Ruler of Amdapor Keep always searching for greater power, but unleashed Diabolos instead... or perhaps some cult to that primalish dreamlord.
The entire dungeon would be less about spamming for myth and more like console RPG sorta stuff. Perhaps during this dungeons your Chocobos gain a considerable Buff to enhance their "Role" whether it be DPS, Tank, or Healer.