Why are seeing pages with the name set to "Placeholder-for-page-of-type" appear in the page structure?
When and why does EPiServer create these?
Those pages is created by copy, import/export or mirroring. They are created when there is a PropertyPageReference inside the package that points to some page that is not part of the package. This behaviour where changed in cms6 where those what we call prototype pages no longer are created.