Ok, if this has been discussed before, I don't remember it.
Why only Derek's room? (well closet, since it was there pre-Derek). Was it purely to get the Arieth photos? And some Tremolo/Sarah photos too, I guess, but surely there wasn't that much action in there. They all have their own rooms. (On that, why wasn't Jacob in his own room?)
What other purpose could it possibly serve? There is 1000 better places someone could put a camera in the mansion, to get much better stuff than getting videos of Derek pleasuring himself, now he's taken over the room.