The SRD is for Flight Planning purposes only. We do it slightly differently in the real world
Traffic via MAC for Glasgow will of course flight plan via TRN and the TMA controller will normally provide an acceptance level at TRN for the flight to the West Coast controller tactically before the flight routes on to LANAK or is vectored for final approach.
However, if holding is taking place for Glasgow traffic then the holding fix and clearance limit point for the West Coast controller is FYNER. There is little point in feeding further traffic from the West into the already busy TMA airspace if there is holding in progress. By issuing a tactical reroute along N553D then it gets the aircraft out of the TMA traffic count freeing up a slot in each of the two TMA sectors involved, and West Coast sector can liaise the aircraft's approach direct with Glasgow Approach cutting down on co-ordination requirements with the already busy TMA controllers. This rarely happens, but it is the procedure available if ATC need it and hence the reason why N553D was put in place when the revised TMA airspace and procedures were drawn up in the early/mid 90s.
The FIR arrivals via ROBBO are for traffic not planned along the ADR structure, e.g from Islay or Eglinton perhaps. Normally their clearance will be at or below 6000' and issued by Glasgow Approach. There may be a wait if Glasgow are on Runway 23 and have a lot of departures since they have to ensure separation.
The use of N553D is not in the SRD because it is not the Standard Inbound Route, just one which exists primarily as an ATC tool .... though nothing to stop anyone filing it if it fits in to their plans