Hi Jake!
The Aft Well Deck was covered as one of many accommodation improvements made over her sister ships, and was done to even out the amount of open and enclosed deck space among the classes.
As we see on Olympic and Titanic, there is plenty of open and enclosed deck space for First Class - Boat Deck A Deck Promenade, there is some open space but mostly enclosed deck space for Second Class - B Deck Promenade, and Third Class was limited to only open deck space - Fore & Aft Well Deck, Poop Deck.
The Aft Well Deck on Britannic was covered to create more open deck space for Second Class and enclosed deck space for Third Class.
Best regards,
Brian