My best explanation is that some people believe that project management can be completely oblivious to the nature of the work to be done, and still be useful.

My best explanation is that some people believe that project management can be completely oblivious to the nature of the work to be done, and still be useful. To them, maximizing their ability to make choices about the way work will be done means they have more ability to take the good decisions that will lead the project to a successful end.