I find it pretty useful on the iPad, it basically adds an extra window UI element on the bottom corner of every window that you can use to rescale the window to some different sizes. Adding a few windows together has been useful for workflow sometimes, and it’s been useful when apps that aren’t optimized for the iPad are running because their little windows are floating along with other windows instead of in a big black void. I think I turned it on a few months ago and haven’t really had any reason to switch back (I like being able to see my desktop background too).