I really don't know. It looks like a steward service and storage area since it is common on the passenger decks. It is also a blank space off of the Promenade and Atlantic decks as well, so it could be a vertical service and access area between the decks. It certainly isn't a disco or anything that would make noise. I had a cabin very close to there on the Legend's Panorama deck, and walked by it often. It seems it was some kind of storage when the door was open.
It seems odd they would design the ship that way. I mean using outside/balcony cabin space as a crew area. I am going to make a note of this oddity though. The next time I book a balcony on the Spirit Class, I may want one of those balconies.