The only thing it was designed for: recon. It's a spy plane. It has cameras and electronics up the wazoo. You fly it high and fast over a target, collect the info you need, refuel it and change target. Unlike a satellite, you can do that many times per day.

Against the USSR it had become risky because missiles could reach it? And satellites worked great during the cold war. But in a scenario where things got hotter and you wanted regular fresh intel from deep within enemy territory, the SR-71 could be irreplaceable.