Placing the call to the impl->Purr inside the cpp file means that in the future you could do something completely different without having to change the header file. Maybe next year they discover a helper method they could have called instead and so they can change the code to call that directly and not use impl->Purr at all. (Yes, they could achieve the same thing by updating the actual impl::Purr method as well but in that case you are stuck with an extra function call that achieves nothing but calling the next function in turn)
It also means the header only has definitions and does not have any implementation which makes for a cleaner separation, which is the whole point of the idiom.
