Why maintain an independent air force?

What is the point of an independent air force instead of only army and fleet air arms?

Its primary role is strategic air warfare and airspace control, distinct from the aviation organic to armies and navies.

The original case for an independent air force rested on strategic bombing and the desire to preserve air-minded thinking outside land or sea priorities. An independent service can concentrate on defending national airspace, contesting an opponent's airspace and applying air power at the strategic level. It can still support armies and navies, but those services retain aviation tailored to their own operations. Army rotary-wing forces and naval carrier aviation face missions and environments that a strategic air arm does not replace. The case for a separate service ultimately depends on the nation's size and resources, but the doctrinal distinction remains clear.