Why Royal Navy destroyers stepped down aft

Why did Royal Navy destroyers use a raised forecastle and a deck that stepped down aft instead of a flush deck?

The high bow improved seakeeping, while the lower after deck saved weight, reduced topweight, and helped torpedo operation.

The raised forecastle answered North Sea and North Atlantic wave conditions. Designers wanted as much height forward as practical so the bow could meet heavy seas without being continually swept. Stepping the deck down aft saved structural weight and topweight while reducing the height from which torpedoes entered the water. A flush deck could offer useful volume, but only at a height and weight the destroyer could safely carry. The break was a deliberate compromise among seakeeping, weapons, and stability.