What aircraft gain from more efficient engines

How do more efficient engines improve an aircraft?

They let designers trade lower fuel burn for range, endurance, payload, a smaller airframe, or speed.

Engine efficiency gives designers freedom rather than a single performance benefit. Lower fuel consumption can extend range or endurance without changing the rest of the aircraft. Designers can instead keep the same range while carrying less fuel and using the released weight and space for passengers or cargo. The same gain can support a smaller airframe or be traded for speed, so the benefit depends on the mission.