If NASA needs an extremely fast, high-altitude aircraft, it would be much cheaper to bring SR-71s out of retirement than to build completely new aircraft from scratch. As for why they’d want it… Maybe they want to experiment with using it as a space launch platform, getting it very high and up to speed and then launching a (relatively) small rocket from its back to launch that rocket into space. Maybe they have a new engine design to operate at very high altitudes and speeds, and they want the SR-71 as a test platform to mount those engines on and test them in real world conditions. Maybe they just want to do some upper atmosphere/ionosphere measurements, and the SR-71 is a handy way to get there. Maybe they want to try operating a telescope from it, using it as a way to get most of Earth’s atmosphere out of the way and have less distortion/interference. (There already is such a telescope in operation, based on a commercial airliner, but the SR-71 could go higher and presumably do the job even better.) Maybe now that they’re barred from running climate research satellites, they’re looking for an alternative, non-satellite based option they could mount instruments to and take the same measurements from. Or, most likely, something else I haven’t even thought of.