The first use that comes to mind is a hypersonic cruise missile or unmanned bomber.
Intercontinental missiles are also hypersonic, but their re-entry vehicles are gliders with limited maneuvering capabilities.
The Russians have developed and used hypersonic air-to-ground missiles in Ukraine, the Kinzhal. It is very difficult to intercept a missile at these speeds, and almost impossible if the missile is maneuverable. This has re-launched research into this type of missile and propulsion system.
Russia is ahead in this field. The USA, China and France have carried out various experiments (mach 5 to 25), but do not really have combat-ready hypersonic weapons.
One of the great difficulties of hypersonic flight is that you generally need one engine and one stage per flight regime. The majority of demonstrators operate according to the following scheme:
- a rocket or aircraft propels the vehicle to supersonic speed, so that it can start its engine.
- The vehicle accelerates to Mach 5. The vehicle starts its hypersonic engine and detaches from the supersonic stage.
- Climb to the edge of space.
- Descent and acceleration.
- Detachment from the hypersonic engine and final gliding approach. The fireball moving off at mach 20 kind of approch.
The engine shown here would enable to have just one engine for supersonic and hypersonic flight, greatly simplifying the vehicle. The fact that it's an engine and not a rocket is important, as it means that the thrust is much longer and can potentially last for the whole flight. At these speeds, it's largely the engine's thrust that makes maneuvering possible, and this is what distinguishes these vehicles from the hypersonic glider/reentry vehicle found on intercontinental missiles.