There is no practical use, because in practice, fixed-wing aircraft cannot fly horizontally in the mesosphere (50-85 km / 31-52 mi) anymore. The Kármán line assumes it, stating that airflight turns pointless at 100 km because stall speed reaches orbital velocity.
While this is true in theory (for altitudes 52-62 mi (84-100 km) depending on aircraft, latitude, and solar activity), a spaceplane cannot keep flying horizontally at lower altitudes anymore, because when trying to do so at too high altitudes it would need to go at speeds that would heat it up too much, due to aerodynamic friction and supersonic flight, making leveled flight in the mesosphere impossible without permanent heat protection from all sides. An air- or spaceplane trying to fly horizontally has to go faster with altitude as air gets thinner. At a certain altitude it would reach the point where it would either have to fly so fast that friction would produce enough heat that would damage it when unprotected, or when flying slower it would stall. This point is reached around the stratopause (31 mi / 50 km altitude) so a plane flying above the stratopause will necessarily have what it takes to be a spacecraft.
History proves this as there never has been a plane that could reach the mesosphere only but not go higher. Planes that could reach the mesosphere (such as the X-15 and the SpaceShipOne, Two and Three) all went higher into the thermosphere or are planned to do so. And the record for the highest leveled flight is currently set by the unmanned AeroVironment Helios plane which flew to 18.3 mi / 29.5 km, well within the stratosphere.
Therefore, in practice the Kármán line is useless as there won't be planes that will attempt to fly horizontally at say 70 km (43.5 mi) without going higher, unless one would build such plane that can't go higher and make them have incredible heat protection if one would attempt that for some reason. But at 70 km altitude, "flying" would mostly be gliding and propulsion barely needed anymore to reach a faraway goal.