I can't believe why would anybody use a fullscreen window (isn't that a contradiction?), apart from a presentation or watching a movie, of course.
In the old days, i had a Classic Mac, with the tiny 9 inch monitor (512x384 1-bit pixels!). It was like looking at the interior through a hole in the wall; but good program layout, together with great design of each element, made it quite usable with several windows around, all of them with some corner still visible. 'Muscle memory' at its best. Lots of small details helped a lot, like how each directory stored the size/position of its window, so it really mattered when you took a couple of seconds to put each window in the best place.
Also, there wasn't a "maximize" button, there was a "zoom" button, which wasn't supposed to expand to fullscreen; it expanded to "the nearest possible to the ideal size". On most document windows that meant full height, and page-size width; but directories with few file on it actually shrink to a neat box without scrollbars. That encouraged good file hierarchies, with a well-defined use for each directory.
Windows programs never got that right, at first they were all designed with single-use in mind, so the 'maximize' button made sense. After GUIs took over, i think it was lazy developers that didn't take the time to create good, well designed, interfaces so they fill so much of the screen with seldom-used icons and tools, that users need all the space they can get just to work on a single thing.
That negates all benefits of windowing. The user is almost never multitasking, just switches from one full screen app to another.
The worst case is also too common: web designers that assume that since it's ridiculous to use a screen less than 800x600, then it's a waste of screen to require less than 800 pixels across. but i want to see other things!
any window that covers more than a third of my screen is an eyesore to me.
any webpage that requires more than half of my screen width will be closed ASAP.