British submarine tubes and ambush tactics

Why did the British put external torpedo tubes on Second World War submarines, especially D-class and similar boats?

The extra tubes served an ambush-predator doctrine: fire a large first salvo, then leave or use aft-facing tubes while being hunted.

The external tubes reflect a British submarine doctrine centered on the first salvo. T-class boats were meant to fire a large spread, escape the counterattack, and later ambush from another position. Aft-facing tubes also gave a chased submarine a direct weapon while it was trying to get clear instead of reloading under pressure.