Isn't it because the connecting / transit pax don't clear immigration before getting to the departure lounge ? Without the photo check someone could arrive on an International flight, with an onward non domestic connection, get to the Departure lounge via the connecting route, swap a Domestic departure flight boarding card with an accomplice and then arrive at the other UK destination without passing through any border control ? The photo check prevents this. Meanwhile the accomplice takes the onward non domestic connection....
maybe ?