You can mount a lot more equipment a lot higher up on a balloon, and it can float places you can't legally drive. The, in theory, purpose of such a balloon is

You can mount a lot more equipment a lot higher up on a balloon, and it can float places you can't legally drive. The, in theory, purpose of such a balloon is going to be something like satellite surveillance but low and in the are long enough to use certain types of sensors that would not work that high up, or that can now penetrate much further into the ground. Not that I really think there's much to be worried about, as even if this is the case its more "nations who's only war will be miserable nuclear fire pretending there's a chance it could be others", but there is absolutely a plausible reason for these.