GNOME bug 150897 points out that as things stand you can’t bind naked modifiers to actions– for example, you can’t have it so that pressing alt on its own takes a screenshot.
The reason this might be useful is that pressing super (which is generally the key with the Windows logo on it) on its own brings up the main menu in Windows, and it would be nice to allow it to do so in GNOME as well.
Of course it could also be done by making the key not be a modifier, but then you would lose the ability to make super-M or whatever do something.
Your chronicler can foresee having to switch away from their trusty IBM Model M to test this one.
Photo © frohner-1, cc-by.