They do it for the slow trains, so that people wanting to go to the real destination don't spend an age getting there:
PAD-RDG fast: 29m non-stop.
PAD-RDG slow: 1h 12m with 11 intermediary stops.
PAD-OXF fast: 57m with 2 intermediary stops.
PAD-OXF slow: 1h 58m with 18 intermediary stops.
I...