As I mentioned previously, part of the reasoning is to allow guards unhindered access to the first set of passenger doors, in order to be able to call at the various stations in the west which have short platforms and at which the guard is required to open and close that single set of doors. As I suggested earlier, this would explain why it involves both ends of the train, which clearly would not be necessary if it was just about providing a staff seating area. It also explains why a similar arrangement is not required on the Turbo fleet still operating LTV routes, where there are no such short platforms, or on other 'West' traction such as the 158s, where the guard has a vestibule behind the cab from which to operate the doors.
On GWR's 80x fleet there was initially a system of taking the first class kitchen car out of public use to allow staff to travel where required as part of their duty, however that was superseded a long time back now by a small number of seats being permanently reserved at the far end of Coach A, when it was decided that the space in the kitchen car was once again needed for first class passengers. So this isn't just staff being precious about expecting somewhere nice to sit, there is a reason for it!