I think there was just a preference to having a guard at the end of a train, where possible - and generally only one coach beyond the one with the guard’s compartment, which was permitted to run with the guard’s compartment at the front (so, brake 3rd and compo, as seen on many GER branches). In any lengthy rake, where there might be reasonable parcels/smalls traffic, then a brake coach at each end sorts everything out nicely: GWR van 3rd, compo, van 3rd branchline sets are a classic example, albeit short and only 3 coaches long. A Market day 3rd could be simply added outside the set without need to remarshal the rake. The EWJR had brake 3rds with a central guard/luggage section, with a 3rd class compartment either side, and generally ran with the coach sandwiched between a third and a compo, which is why I have three such coaches and no more than that at the moment!