engine performance
At high altitudes,the air is thinner.
1. thermal efficency greater at higher temp differentials, engine is a heat pump the hotter the efflux the more efficient the burn.
2. compressors work better at high rpms, hi altitude flight allows more rpm without exceeding thrust/airframe speed limitations
3. reduced friction/drag at altitudes allowing lower output at altitude, while retaining a set mach/airspeed. better SFC