> in what circumstance
Precisely in the one that brought this into topic. It's nice to not have to be so careful with how the mouse moves. When it gets in the way, I can just flick it off, not worried about my stuff changing focus. It also greatly decreases the annoyance of accidentally touching a touchpad while typing.
You do it by moving the mouse. It makes the mouse more useful because you can now focus things with one less click.
On the other hand, in what circumstance would you want to move your hand to the mouse, move it intentionally over another window without clicking, and _not_ focus it? What would be the point in moving the cursor?