Aerial reconnaissance is not all about visual images and cameras, there's electronic intelligence gathering done from the air and space as well.
Heading for a TL;DR but anyway an example, a bit of speculation:
Lets say a major power 🤐that might get into conflict with the US wants to develop countermeasures to, say, for example, the US's leading air to air fighter, the F-22, most specifically counters to it's radar and maybe also countermeasures for the radar on it's air to air missile of choice, the AIM-120.
To develop those countermeasures the major power 🤐really needs to analyse all the characteristics of said radars such frequencies the radars use in different modes, scan patterns if any...there can be lots of useful stuff gleaned if you can get just "listen" to the raw data..
One way of gathering the info might be to equip a relatively innocuous high flying balloon with lots specialised radio receiving equipment plus the kit to transmit the received data back home live.
Fly the balloon slowly over the US and wait to be "looked" at, both visually and by radar and other means, such as fighters such as the F-22.... Potentially you've opened up an electronic intelligence old mine.
The unnamed major power 🤐would probably consider it an electronics intelligence jackpot if the US decided to use an F-22 plus AIM-120 to knock down a suitably equipped balloon..........there would be lots of lovely data to analyse, so it's interesting to note that the US has used the less complex AIM-9 missile to knock down the balloons so far - the reason for using that missile might not just be one of economics.
All that's just speculation but it's an example of why using a balloon could still have a niche use when it comes to gathering some forms of intelligence.