It's useful when you have a large screen with multiple windows that are tiled and dedicated to a single work function. For example a terminal on one side and a browser on the other that's dedicated to specific work. You may then have another virtual desktop dedicated to messaging, video editing, whatever...

In well designed WMs like i3 each desktop is typically bound to a key for instant switching, unlike the full screen atrocity of macOS where there is an unremovable delay when switching.