A fleet in being as an active strategic posture

Has a fleet-in-being strategy ever helped win a war?

Yes. An outnumbered force can remain active at a distance, preserve its striking potential, and deny the enemy an easy opportunity for decisive destruction.

An early British-French example shows an outnumbered Channel force avoiding close blockade while continuing to patrol and watch the enemy. Its admiral deliberately described the posture as keeping a fleet in being. Remaining forward preserved pressure and denied the French unrestricted freedom of action. Staying far enough back to avoid a sudden ambush also protected the force from the decisive defeat its numerical weakness invited.