On another note, on tickets it says "any valid route". What does this mean?
It says 'Any Permitted' on some tickets. This means you can go via any reasonable route. The reason why it says this is because there are often tickets which only allow travel via a certain route because they exclude travel via a more expensive route.
A Penzance to Southampton ticket will cost less than a Penzance to Reading ticket because Southampton is closer to Penzance. Logical, yes?
Then consider there are two 'reasonable' routes to Southampton from Penzance. You can change at Westbury, Exeter or Bristol and head down via Salisbury. Alternatively you can stay on the Great Western Mainline and change at Reading. This means you cover a far greater distance than you otherwise would.
As a result, the default ticket option for Penzance to Southampton will have 'Route: Salisbury' on it. Meaning you MUST travel via Salisbury for that price. You can, if you wish, request a Route: Any Permitted ticket which will permit you to travel via Reading, but cost much more as a result.
Where there are a number of routes with no real advantage to any of them, Any Permitted tickets are usually issues by default.
The 'Route' section can also be used to restrict travel to one TOC only - for example if they are running a promotion. It is cheaper to travel to London from Birmingham via Chiltern Railways for example - it takes longer than Virgin Trains and the train isn't as nice but by offering a Route: Chiltern Railways ticket that operator can offer you a cheaper fare by you only using its trains.
I do agree that the ticketing system is bizarre and confusing. It is for example cheaper for me to travel from Plymouth to Worcester by purchasing a return to Tiverton Parkway and a Freedom of the Severn and Solent 3 in 7 day Rover (Allowing 3 days of unlimited travel from Tiverton up as far as Great Malvern, Cardiff and Portsmouth on the South coast!) than it is for me to simply purchase a return to Worcester. A great deal for me, but should you really need specialist loophole knowledge to get the best deal?