Deck and belt armor are being added partly to balance the hull rather than solely for protection. That forces a protective system to perform the separate naval-architecture task of controlling trim and weight distribution. An explicit ballast control would let designers correct the balance without distorting the armor scheme. It would also make stability decisions visible and leave protection to be chosen for combat value.
Notable Point · 58:44-59:33
Why a warship-design game needs ballast
The game forces armor to serve as ballast, prompting an argument for explicit weight-balancing controls in ship design.