Windowing was a core part of classic MacOS applications, the application was decoupled from its windows, since the application was a composition of windows. It's very different from the MS Windows or tablet UI paradigms where an application and its main window are the same thing.

For example, Photoshop, your photo document was contained in a window, your tool palettes were windows, etc. If you were viewing a reference image, you could have that open in a separate window alongside the image you were working on, rather than having to switch tabs or switch windows.