Autofocus controls work well most of the time, but not all the time. When should you override it and use the manual focus control? When the camera can't decide what to focus on and "hunts," that's when you want to use autofocus. Similarly, when the camera focuses on a subject that's not the correct subject, you'll need to turn off the autofocus and turn on manual focus.

It may be helpful to know that professional camera operators rarely use autofocus. I am not suggesting that you avoid your autofocus feature; just be ready and able to turn it off if the situation arises.