This makes a difference when something doesn't work, as you might not see the error message, and realise why it didn't work. 23.6 Window focus, follows mouse. I used to prefer the top window to stay active, now I prefer focusing with the mouse and not having to click. Another option, is to have the window you type into, come to the front, or to stay where it is. In addition, within a window, the current focus may require a click to get the text cursor where you want it to be.
Intelligent applications feel more natural, as they do that automatically.
Most applications have an thicker border around the widget that holds the keyboard focus. For example, a form containing several fields, has the keyboard focus in one of the fields, not necessarily the field underneath the mouse. Comment it all out. You could of course type it manually or add it to a menu somewhere. I used this command, when my old mouse developed a faulty button.