Since you said "default" behaviour, then you know that Option-clicking the green button gives you the traditional Zoom (which is not always the same as Maximize) behaviour. Some apps do Maximize, others just Zoom to accommodate the content shown in the window.
Full-screen is very useful with multiple Spaces or desktops. Four fingers swipe left-and-right to switch spaces. It's a very useful workflow. Or multiple monitors to achieve the same thing. Combine that with split-screen view (each of which is always "full-screen" in its own rightful size), and you have a blend between full-screen and multiple windows in the same space.
Overall, it's a very powerful and flexible window-management feature set. Spaces used to confuse me to no end, but they really cleaned up their behaviour.