"Supercruising" means flying supersonically for at least 20 minutes. In a fighter aircraft the salient requirement is often not the time that the plane can fly supersonically but the distance. The Mig-31 has a combat radius of 450 miles at Mach-2.35 and a top speed of Mach-2.85.

If you need a fighter to fly 500 km and engage a target, why wait 25 minutes for a super-cruising mach-1 fighter when a Mach-2.3 craft without super cruise can get there in 10 minutes?