If you are switching between 3 applications placing each in turn maximized on your display indeed I would see no benefit from virtual desktops in this instance

If you are switching between 3 applications placing each in turn maximized on your display indeed I would see no benefit from virtual desktops in this instance however the benefit is proportional to the average work needed to switch between states * the frequency of such a switch. With the complexity of keeping track of which together rising with the number of windows. If S1 consists of a single window maximized and S2 a different window maximized the cost of switching workspaces is just as high as clicking on say a taskbar or hitting alt tab. Let us consider a very simple stacking window manager with 2 workspaces and a task bar. Let us first disregard the second workspace. Suppose one has taken the trouble to size 7 windows to fit on 3 monitors. First you drag one window to the left-hand side of the leftmost monitor. Then we drag one side then the other. Repeat 7 times over for each window to be placed on each monitor. Now suppose we only have 3 applications we ought to be able to see together separate from the other 7. This is easier as they can be dragged to a display and maximized. Now consider the hassle to recreate the state repeatedly. You don't need to resize it but you have to manually select first 7 then 3 then 7 then 3 applications to bring them to the forefront. Now what if you want to show 3 windows from the rightmost monitor then put them back. Now we are stuck resizing each in turn. Now let us fully utilize the workspaces and separate the states on different desktops. Suddenly the switch between S1 and S2 is a click. The operation of moving the applications displayed on one end of S1 and showing them on the other end of S2 is of course just as much of a hassle as it ever was because workspaces encompass all monitors at once. Now let us lastly switch to using per monitor workspaces. Now the last operation is as trivial as the first at the expense of making the process of changing all 3 at once more expensive. To decide which makes sense we would consider how frequently we want to do each operation. If we find that we frequently need to switch multiple monitors at once we can trivially add that back but not the reverse.