Once upon a time, in a land not so far away....
There used to be Advance fares (Apex / SuperAdvance then). They were designed to fill empty seats.
The capacity management department would look at see that the 10:00 from Manchester to Euston regularly left with 100 empty seats. So they would offer most of those seats to people who could book in advance.
The trick was to attach rules to advance tickets to stop people who would pay £100 paying just £10. Those rules were "book 14 days in advance", "no refunds", "train specific", "no alterations".
The would also use quota controls to encourage people to travel on other trains. So the 12:00 Kings Cross to Inverness could be £50 to Edinburgh - but the 11:00 and 13:00 departures have £30 tickets available. This encourages people travelling to Edinburgh to use other trains and save the direct Inverness service for those who have only one direct train per day for stations north of Edinburgh.
To complicate matters (I'll use the 12:00 ex KGX as an example)...
You could have
10 x £10 tickets from London to Aviemore & Inverness
10 x £30 tickets from London to Edinburgh
10 x £35 tickets from London to Newcastle
10 x £15 tickets from York to Inverness
In this example, the cheapest is from London to Aviemore/Inverness - to compete with the airlines.
It's cheaper to buy a ticket to Inverness than it is to Newcastle - which is a reason why Advance tickets are not available for the journey to be started or finished short.
Also, London to Inverness is cheaper than York to Inverness, so someone joining this train are York should be charged a new ticket !
They could also put a journey restriction on. So the 0700 from Manchester could have Advance fares - but not to London (so Milton Keynes for example).
Then, along came the private railway. Advance fares were no longer just about filling empty seats but about making money. Some bright spark realised that the £100 fare from Manchester to London was split. Virgin would get £50, Northern £5, London Midland £20, Chiltern £15, TPE £1, East Midlands Trains £2.50, Great Western 50p and Cross Country £1.
So they offered an advance at £30 valid on Virgin Trains only. Two of these singles are £60 and this pushes up the money earned by Virgin by 20%. So now it's more profitable to offer an Advance than a Off Peak Return !
Trains are often full and standing - yet 50% of the people on board have advance tickets !
Imagine a hotel that was regularly fully booked and they only charged £10 per room per night !
There can be up to 5 bands of Advance fares. The amount in each band is commercially sensitive.
Try flying easyJet in six months time. Book today and you can get a fare for £20. Then it goes to £30, then £40 and up and up and up. The railway are just doing what airlines do. However, airlines aren't perceived as having "bands" - it's just that the fares go up the busier the plane gets./