Just my random thoughts on the key; lots of assumptions and very probably wrong..but!

a) Assuming it was the artifact made by Azem, it's abilities don't really make sense, unless it has something "else" to it; A guess might be Space-Time travel/teleportation without aetheryte (as seen in Ascians); With ancients, they have aetherytes and walk/fly/arrive at the main gate/move normally in Elpis, so one might assume they don't have ascian-bampf powers yet.

b) The shard-travel might be an unintended effect of what the key does. It could have been originally designed to traverse alternate dimensions or the inter-dimensional rift. Assuming the cross-travel of characters like Lightning or Shantoto are canon, access to other worlds might have been the goal. (I hope this isn't the case, and I hope the key isn't just thrown away a la "oh so that's how you get to the FF11 raid, lol")

c) because story reasons, it might be something strongly connected to Meracydia. Cargo-cult worship or Azem or something.