Let's say I have just two monitors. That box being checked means I have spaces A1 A2 .. An and B1 B2 .. Bn, and monitor A can display any of the "A" spaces and monitor B can display any of the "B" spaces. But if I want everything from B2 to get displayed on monitor A, I have to create a new space An+1 and drag everything over one window at a time, like a cave man.
Unchecked means I have spaces AB1 AB2 .. ABn, and switching spaces switches both monitors at the same time! So if I want to see the A side of AB1 and the B side of AB2 at the same time, I'm out of luck.
Xmonad workspaces allow me to do any of the above behaviors (oh, and it switches workspaces instantaneously, instead of this left/right spatial thing macos insists on, I guess because of the trackpad swipe gestures). One of these days I need to try to make Hammerspoon fake workspaces in a way that works for me, because (like the OP) this is by far the least comfortable thing about working in macos for me.
