These maps are often supposed to be schematic rather than geographically accurate. The Tube map being a prime example.
The tube map displays all TfL-sponsored services, and some others. It's certainly not a complete document by any means, but it's rather more complete than this one is.
The only reason I can think for wanting this information to be easily displayed in this format is, for example, if you need to know if your train will be a Turbo so you can fit your mobility scooter or whatever onboard, and it has been taken out of context by mistake.
That's just an example, I have absolutely no idea what GWR's actual policy on carrying mobility scooters is, and I can't be bothered to look it up at present.