Empty missile cells preserve cheaper future growth

Why build more shell or missile capacity into a warship than the navy initially has weapons to fill?

Weapons can be bought later, while adding magazines or launch cells after construction is much harder and more expensive than installing the capacity from the start.

A navy may not have the budget or stockpile to fill every cell when a ship commissions. That shortfall can be corrected through later weapon purchases. Structural volume, launch equipment, and supporting services are far harder to add after the hull is complete. Empty capacity therefore preserves an affordable route to wartime or future expansion rather than representing wasted space.