Illuminating an area with a somewhat bright flash produces more brightness on objects closer to the flash. By arithmetically subtracting a non-flash-illuminated frame from a flash-illuminated frame, you can extrapolate what the scene would look like if the flash were the only illumination. This could be useful to provide illumination for actors, etc., that isn’t visible in a photograph or motion picture. This probably isn’t useful for confidentiality, since brighter areas of the subtracted background will have larger amounts of noise in the subtraction result, but it could provide dramatic lighting effects, even permitting after-the-fact lighting adjustment.