> "Surveillance? They've already got cameras for that."
Not quite. The usual sort of cameras are at fixed points that can, at most, swivel around. To achieve full coverage requires many cameras.
Conversely a drone camera may move about, locating all available spaces and surfaces, and imaging them from all angles. And this may be done with far fewer cameras, achieving the higher security levels of human guards in the process.
Reasonable cost, no big human component to be paid or get tired or just phuck-off, and comparable or better security. I'd say this is the future, except for the fact that a hack attack might disable the entire system. Human guards are resistant to that sort of thing.