With exception to single window utility programs, Mac windows have always truly closed with the resources taken by the represented document being freed and all that. The windows weren't hidden. It's just that closing the window ≠ quitting the application… the program can remain in memory even if it has no documents loaded.

This serves a couple of purposes: first, documents open more quickly (particularly when the program is loaded from a slow spinning HDD, floppy, etc) since the program doesn't need to be reloaded, and second, new document creation flows and non-document functions can be accessed without having a document open or requiring the developer to create a bespoke "home screen" UI that serves that purpose since the full menubar is accessible as long as the app is foregrounded.