Greybeards who still used SunOS because they didn't like the Mac/Windows way of doing things.

It enables things like:

1. Found a web tutorial on how to do something in *nix with a bunch of shell commands interspersed between paragraphs? Keep a small terminal open in a corner on top of the fullscreen browser, read the article and actually click-and-select commands and then copy-paste in the terminal by just moving your mouse back and forth between the applications.

2. Looking at a scanned PDF and want to do a bit of math to confirm the numbers you're reading? Load up a calculator and type into it when the mouse is hovering over, go back to scroll the PDF. Easy peasy.

Basically by separating the actions of window positioning from using it, you can actually use more than just fullscreen windows one at a time.