It's just free money for what is usually mild inconvenience
DR15 came about because 20-30 minute delays are not “mild inconveniences” when we’re talking about commuters who are paying a lot of money for 20-30 minute commutes.
The political pressure was needed to make operators actually operate the service they said they would. Back in the days when I was commuting into London from Hemel it was literally one trip in ten which arrived on time. A good half were heading towards 10-15 minutes late; LM had no financial incentive to fix it, so they didn’t. I kept a spreadsheet.
Even back when it was a 30-minute threshold I was getting several hundred pounds back each year just through London Midland’s incompetence. And it was nearly always their fault. A common one was to cancel the Tring terminators, and with a 30-minute frequency why wouldn’t they? They save the cost of running the train and they don’t have to pay out a penny in compensation. Half an hour standing on Apsley station on a January morning was always a load of fun…not.
This is the thing to bear in mind: in isolation a 15-minute delay is no big deal. But when it is happening 3-4 times a week, as it did with me when I was commuting with LM, it is a big deal. On a bad week I’d be losing two hours a week to their incompetence.
It’s also not “free money”, it is compensation for the railway’s failure to deliver the service paid for. Delay Repay is tied to the amount paid for the ticket, so the compensation can’t exceed the amount actually paid to them in the first place.