I’ve always suspected this is to accommodate for the last District line train to Richmond. However I’ve never checked the timetable to confirm when that is. This potentially allows users to reach this destination if they miss the last train at Hammersmith.

Similar to how the last westbound central line train to Ealing Broadway holds at White City to allow the final westbound train of the night (towards West Ruislip) to transfer the last stragglers from the city