The Expanse

I really must ask, what the heck is the key used for at the end of the new player dungeon?

That key opens a door at the end of the non-combat training area. There is a portal inside there that takes you back out of the non-combat training area. but keeps you inside the New Player Dungeon (in case you want to work on your combat techniques more).

Oh you mean the door that's been open all along and doesn't need a key. Alright.

@Kaitox This feature is no longer part of the quest and had been sidelined for redevelopment. However, as a shard owner yourself, you will be well aware that sometimes other features are given greater priority, particularly when the rate of production is as prodigious as it is at the Expanse.