I think there is a point to this, after all
Arriving at LGW yesterday from AMS I realised why they have to do this. If they didn't - given the way things are set up - arriving international pax could bypass immigration.
What you would do is arrive on your international flight having arranged to hold two 'onward' boarding cards: one for an international connection and another for a domestic. Then you would go through international connections (which deposits you back airside) using your international boarding card, and use your domestic one to get on a domestic flight, and never face passport control. The need for a barcode/photo to get to the domestic gate stops you doing this.