How paired high- and low-pressure turbines save fuel

Why was the paired high- and low-pressure turbine arrangement used in ships such as Warspite more effective?

It improved energy efficiency by letting low-pressure turbines provide cruising power while high-pressure turbines supplied top speed.

The arrangement uses the low-pressure turbine efficiently at cruising power and brings the high-pressure stage into play for maximum speed. This makes the propulsion plant more economical across the ship's operating envelope than treating the turbine sets as separate power units.