You want to test your ability to get physical payloads in place, e.g. in the future the drone could carry a bomb or something else clever (listening devices? Dropping gallium on aluminum airplanes? Etc)
You want low angle shots?
You're not well funded enough to get good satellite photos?
Hypothetically it might act as a more subtle form of sabotage, the goal being to bring down a plane in the air. It might also be harder to cleanup/patch than a bullet hole if detected. I don't think it's a serious threat though and is really acting as a stand in for "fancy forms of sabotage".
Don't satellites only occupy a given area once or twice a day (if not geosynchronous)? If you know the pattern you can hide assets before it gets in view. A drone can do surveillance at any time.