One specific example is if your contract has a termination period, which is pretty common, at least in my part of Europe. If you simply stop paying, you are denying the other party N months of revenue (your cancellation period) that you are contractually obliged to pay. You are now defaulting on your payments and will likely pay additional fees.
But if the contract has no minimum term (or it has since passed) and you've made a reasonable effort to attempt to cancel with no success, it'll now be on them to recover the money through legal means which would require them to explain to the court why your cancellation attempt was ignored, demonstrating their bad faith in the process. That's not something they want to do.
In the end I paid it though, it was only 20 euros!