I think Brucey is right here in explaining about the Routing Points.
For a different area I have found Easement Number: 700372 for GreaterAnglia services on the Shenfield line which has this explanation:
Customers travelling from Brentwood via Gidea Park in possession of tickets routed "Any Permitted" may travel via Shenfield. They may not leave the station. This easement applies in both directions.
I think prevention of leaving the station is a reasonable clause also. Shenfield is a staffed station, has a Pumpkin, Toilets and Waiting Rooms.
That being said I have broken the journey at Shenfield on a London Termini - Southend Victoria ticket to pop across the road to an excellent chip shop immediately opposite the station without any trouble at all.
In my opinion the clause is there to prevent those travelling to Shenfield buying a ticket as far as Brentwood (cheaper fare) but travelling onto Shenfield and leaving the station. If you were to take a direct Brentwood - London Liverpool Street it would take forever as it stops at every station on the route. The stock, Class 315s are the most awful things to sit on at the best of times. But from Shenfield - London Liverpool Street. No thank you!
Most trains from Shenfield - London Liverpool Street are next call Stratford or London Liverpool Street so the added easement is a good offering by the
TOC▸ as it substantially reduces the journey time involved and probably helps ease crowding on services. The trains are also much nicer being Class 321 or Class 360 which have Toilets and First Class Seating.
I don't think we are arguing that the reason for the double back is to break the journey and to avoid paying a higher fare in this case despite my ramblings. But for those with heavy luggage, those with travel needs, wheelchairs, poor mobility etc it is a completely reasonable suggestion which may even be valid already. It would be worth having some official statement from
FGW▸ on this.
Here is the map of the current Trowbridge Walk... Google Maps suggest it's just short of 400 metres.
https://www.google.co.uk/maps/preview?hl=en#!data=!4m36!3m27!1m4!3m2!3d51.3202809!4d-2.2140779!6e2!1m4!3m2!3d51.3196309!4d-2.2142879!6e2!2e2!3m8!1m3!1d1403!2d-2.2138042!3d51.3199089!3m2!1i663!2i432!4f13.1!6m2!1m1!1e4!7e0!8m2!1e1!2b1!5m2!13m1!1e4!7m4!11m3!1m1!1e4!2b1!6m1!1e1&fid=0i1