Why would you use a balloon for surveillance?

A Chinese high-altitude balloon was tracked for several days by Canadian authorities, as it moved over the Northwest Territories, northern Alberta and Saskatchewan, before being made public as it entered US airspace over Montana. It was then allowed to travel all the way to the east coast, where it was shot down just off the South Carolina coast, supposedly once it was past the point where debris might hit people or property (a less than convincing explanation in my view). It is hoped that some debris will be recovered from relatively shallow water. 

China, of course, insists that it was just a ("mainly meteorological") weather research balloon that blew off course, but then, who believes anything the Chinese government says any more? It is blustering about the "serious violation of international practice", and "the US's use of force to attack civilian unmanned airships", and rumbles darkly that it "reserves the right to respond further" (which is probably what it wanted all along.

My first question, and it seems I am far from alone, was: why a balloon?. In an era of sophisticated satellite and drone technology, why on earth would you choose to send up a glorified weather balloon?

Well, it seems that there actually are some valid reasons for using high-altitude spy balloons. For one, they are significantly cheaper than satellites. For another, because they operate within the bounds of the earth's atmosphere, it is possible to obtain better quality images. However, they still typically operate above the range of most planes and, because they move relatively slowly, they can often avoid being spotted by radar (other technology and special paints can also help to conceal them). They are more manoeuverable than satellites (although less than drones), and can make less predictable moves if required. They can even spend a long time hovering over one particular area (weather permitting). History shows that they can also be surprisingly difficult to shoot down, although this one was taken out relatively straightforwardly.

So, anyway, yes, there are reasons why a balloon might be used for spy surveillance purposes. Other spy balloons have been spotted over the United States over the last several years (although Trump denies this), and the US itself (as well as the UK) has been investigating the technology for its own purposes. Personally, I'm not sure why the US decided to shoot it down, rather than capture and investigate it, but maybe that's harder to do than I think.