There are a limited number of spy satellites, and their orbits are fairly fixed. If you need to look at a site you have to wait until a satellite passes it (and the people at the site will be able to know quite far in advance when a satellite is coming). And there's no way to delay the pass until the cloud cover clears.
You could potentially hide a satellite from radar observation the same way you can for planes. That's apparently what the Misty program [1] did. Synthetic aperture radar could take care of clouds, though you lose color. Agreed about maneuverability I think.
What kind of intelligence mission benefits from a fast flyby but cannot be performed by a satellite?
The orbits of satellites are known. The target can arrange activities to occur when there is not a satellite overhead. A fast flyby can occur at any time with little or no warning.
Like handling security cameras... you can fire an artillery shell at a security camera, but most of the time security cameras are mission ineffective if you just shine a bright enough flashlight (or laser) at them.
There's no need to cause an international incident by blowing something up when all you need is a bright light for a couple easily predictable minutes.
On the other hand a stealthy hypersonic flyby is probably invisible.
Another military aspect civilians never want to talk about is photo analysis depends on illumination and rando satellite passes can't see into valleys and get messed up if the shadows are weird enough. Just because technically a satellite passed within range of Afghanistan as a whole country, doesn't mean you can see what's happening on the wrong side of a mountain with bad illumination and bad view angle. There's a lot of satellites but not THAT many. Afghanistan used to have boots on ground to launch UAVs but not so much anymore.
Satellites were great during the cold war when the targets were consistent, and the satellites could be put in orbits with good coverage of those areas (eg Russian military bases). Spy planes like the SR71 and U2 that fly high and fast to evade detection or destruction are essential for getting immediate photos of an area that may not have regular satellite coverage.
They probably won’t send in U-2 or MQ-9 to stick ninja swords through the roof of a luxury sedan past rings of S-300 SAM batteries few hundred miles from shores.
I thought SR-71 had some strike capabilities, but I could be mistaken.