I have read some speculation that the greater proximity to the surface allows the antenna array to pick up weaker signals - especially short-distance line-of-sight communications that are used by the military. Even if they can't crack the encryption, just knowing the location and duration of the squirts, and their timing, can reveal a lot.
(from unidentified Pentagon source that spoke to CNN): The balloon's spyware payload, the size of a regional jetliner, had "multiple antennas to include an array likely capable of collecting and geo-locating communications."
There is also this, from David DeRoches, a professor at the Near East South Asia Center for Strategic Studies at the National Defense University in Washington, DC. --
He told Al Jazeera the Chinese balloon shot down by the US could also have been used to “gather information on what kind of signals [the US is] using to track it, so it could possibly identify and classify radar hits … which could be of interest if the Chinese wanted to actually launch an attack.”
Stuff you can't get from a Pine Gap + satellites type setup, IOW.
And also, obviously, the relatively slow speed of a balloon v LEO satellite, and greater proximity will also yield clearer images of ground objects of interest. (and geosynchronous satellites are at an even greater distance, and so their imaging can be pretty foggy)