Some do for their own reasons, in case there is a problem, personal preferences as to who they want to have sex with, don't like a specific ethnicity because of "bad past experiences", etc, etc, etc.
I'd never see one that asks for reasons of anonymity in case I am being scammed prior to a booking. Appreciate Ring cameras and the like are there too, so probably will be recorded in some way, but sending a photo in advance is (for me) something I would never do.