To quote: "Ten minutes! This thing will be over in two minutes! Get on it!"
Supersonic gets you into the fight before it's over, there's no point chugging along, your mates getting overrun (be they ground, naval or air units engaging or about to be engaged by the enemy) and the enemy leaving before you get there.
Equally if you get into a bad situation better to bug out and live to fight another day, supersonic helps in that regard, would you like to be subsonic trying to escape a bunch of supersonic Russkies?