You will need this to gain access to the guardian room to do jewelry combines. Found out the hard way cleaning my bank and destroyed it. Used a reclaim to get it back, couldn't open the door without it. Does not add to keyring.
They key leaves you open the locked door behind the quest givers to access The Font Guardian. Hail the Guardian and they mention something about doing some trials, but thats as far as I got, nothing happened. Maybe for later use? Or need something more?