This is (in many parts of the country) mainly for those tickets where there is only one route possible for all or most of the journey. "Any Permitted" can still be seen on some tickets where this varies from more restrictive routes which would also be available.
A full stop has to be inserted, I believe, because many ticket issuing and vending systems do not allow the Route field to be removed entirely, and in some cases would not allow an Route field to exist without any characters at all.
Some tickets also say "Direct" instead of "Any Permitted", where the most direct route is intended to be used instead of a circuitous one, with extra validity such as the possibility of going via a major station where it could be useful to stop short / break a journey, etc. It would be technically possible to put "Direct" instead of the full stop. However, if there is only one route, passengers might be misled into thinking that the word "Direct" means that break of journeys can't be done by default, that a train which terminates half-way along the route can't be used to get them as far as possible, and so on. A full stop is a bit of a bodge job, but it has no such connotations, it's better than crashing the computers, and does at least look like an intentional placeholder.
