Why would you want manual focus? Any time you need precise control over focus, manual focus is your best bet. For me, a working example would be when I am using ProCapture to photograph birds landing on and taking off from a perch. I set a focus target on the perch where I think the bird will land (see illustration below.) I switch to manual focus and then aim at the target, lock in my focus and then I forget about it. I am going to be pressing the shutter button when birds enter the plane of focus. Since birds move quickly and change direction quickly, and since there is always a potential for the background to confuse the autofocus in those fast-moving situations, I prefer manual focus.