Go from Swindon to London at peak hours costs an extortionate £60.50.
Book the ticket from Swindon to Reading and then Reading to London Paddington costs £34 + £22.20 = £56.20, saving you £4.30.
The train from Swindon to London always stops at Reading anyway and you will spend your journey in the exact same train taking the exact same amount of time and you will stand just as uncomfortably for your slightly less extortionate fee. And as opposed to swapping tickets with someone, this is perfectly legit and not against the terms of service.
There may have been some original sensible reason, but it sure feels like a scam to me.
Also, some airliners (KLM, I'm looking at you), charge you MORE for a single flight than they do for a return flight. When I moved country (and consequently only wanted to book a single), I had to book a return ticket which I simply didn't turn up for, otherwise it would have cost me £500 more. There may be some logic in what KLM is doing, but it feels like a big "fuck you" to me.