US, China trade spying charges: Why are balloons usually sent into air, can they be used for surveillance?

Context:

• The United States and China are currently involved in an extraordinary standoff over allegations of espionage through high-altitude balloons.

High-altitude balloons:

• Balloons have been in frequent use for several decades now, though the first uses go back at least 200 years. They are used mainly for scientific purposes but increasingly for tourism and joy rides, surveillance, and disaster relief and rescue.
• Weather agencies routinely use balloons to make measurements – of air temperature, pressure, wind speed and direction, aerosol concentrations.
• Because of the high altitudes that today’s giant balloons can attain, they are considered useful for astrophysicists and even space agencies. These are relatively clear spaces, much above the heights at which airplanes fly and far below the nearest orbits, about 200 km from earth, where satellites are placed.
• Very often, they offer better opportunities to observe specific parts of the earth, and are also thousands of times cheaper than satellites.
• Moreover, because the balloons are brought down after their job is done, the instruments used are recoverable and reusable.
• High-altitude balloons are tempting vehicles for espionage operations.
• They can hover over an area for a prolonged time.
• Big balloons can carry a few thousand kilograms of payload, which means they can be packed with spying instruments.
• The biggest advantage is the greater prospect of their remaining undetected.