Not even a serious guess
Some think they're for calibrating Chinese satellites. With the resolution that spy satellites are capable of these days, a target that size wouldn't be much of a challenge.
But what if the camera was somehow on the Moon?
I can't think of a good advantage. You could see the whole Earth with one camera but you'd have the Earth phases to contend with.
Too bad Larry J isn't around, I bet he could shed some light on it.