> I think they serve a good purpose once you realise you've stepped out of OOP. You have the idea of a command, or a service, and it does a bunch of procedural things as part of its execution. It only has one public method, so the private methods are a way to describe the steps of the command.

It doesn't have to be a step out of OOP, and their usage is not limited to "Single Public Method Objects". Single use methods are just a convenient and clean way to make complicated code easy to digest, OOP or not.