I think going back historically, overlapping windows appeared on PCs around the same time as VGA (more powerfull platforms had them earlier). The early apps didn't have so many features (and menus) and often had to work together often so it made sense to have them side by side and not to bother closing them to open them again in a few seconds.
Now, it depends on the screen (resolution). Isn't it pointless to maximize everything on a say 23" 1920x1080? I don't. on the side of my browser I often have chat, terminal, even spreadsheet windows. They even overlap sometimes, which I don't mind because its convenient for switching. However, on a 1280x800, I maximise my browser, IDE, etc.