The window metaphor for displaying apps and documents has been the dominant desktop interface since its introduction 40 years ago. It is easy to understand and provides a sense of context. But since then, apps have become more varied and complex, screens have become bigger and new input technology has emerged. Window management has not managed to keep up with the needs and possibilities of today.
If you are just using a single app, the window metaphor is inefficient. You are just using a fraction of your screen, since the desktop in the background is just wasting space. You will sometimes be using your apps in different configurations, requiring you to constantly reposition and resize windows to fit. And if you want to quit a window, you can never be quite sure if you are just closing a document or shutting down the whole app.
Once you are working with multiple apps, it gets really messy. Your windows will get buried behind other apps. Since you can only have one window active at once, your other windows will take up space while essentially being useless. To have multiple windows side-by-side, you need to individually reposition and resize them. All of this results in windows just not being a good interface for multitasking. That was fine for most people decades ago, but is unsustainable for productivity today.
With the next generation of users growing up with mobile devices and fullscreen apps, windows become an unintuitive burden. Apple and Microsoft recognize the problem, but fail to solve it by adding more features on top of outdated interfaces. We need to completely rethink window management with a focus on multitasking and efficiency for modern needs and people.