This is why, if you live in the UK, you'll have noticed that sometimes there's a much cheaper ticket available to you that specifically requires you to travel via some particular station (or e.g. "VIA LONDON" / "NOT VIA LONDON").
The Rail Settlement Plan will assign all the revenue for that ticket to the company that operates the trains you would need to take to perform that journey via the route described. Whereas even if that's the obvious way to make your journey, if any other operator could have got you there via a route in the big book of acceptable routes (roughly anything that doesn't involve going back on yourself or taking an obvious diversion but the book makes this specific rather than a judgement call) then RSP would give them some fraction of the money for your ticket, just in case you took that option.
For an operator getting 100% of a £40 ticket is clearly preferable to getting 50% of a £60 ticket and perhaps strategically better than getting 80% of a £50 ticket for which a rival gets £10 despite not actually doing anything.
Of course this doesn't actually serve passengers in any way, so it's at best an unhelpful inefficiency of privatisation and at worst one more example of how it is wasteful and made things worse. A single National Rail operator had no need of such complexities.
The cheaper routes are often ones that are slightly slower and/or less busy. So the idea behind offering cheaper tickets with specific routing isn't only about revenue allocation, it's about encouraging passengers to use less crowded routes and services. Often it will be the same TOC operating both routes so it doesn't make any difference to them in revenue terms.
The Rail Settlement Plan will assign all the revenue for that ticket to the company that operates the trains you would need to take to perform that journey via the route described. Whereas even if that's the obvious way to make your journey, if any other operator could have got you there via a route in the big book of acceptable routes (roughly anything that doesn't involve going back on yourself or taking an obvious diversion but the book makes this specific rather than a judgement call) then RSP would give them some fraction of the money for your ticket, just in case you took that option.
For an operator getting 100% of a £40 ticket is clearly preferable to getting 50% of a £60 ticket and perhaps strategically better than getting 80% of a £50 ticket for which a rival gets £10 despite not actually doing anything.
Of course this doesn't actually serve passengers in any way, so it's at best an unhelpful inefficiency of privatisation and at worst one more example of how it is wasteful and made things worse. A single National Rail operator had no need of such complexities.