As others have said, it depends on the camera.
Manual adjustments allow you to catch movement, increase depth of field, darken the background, tighten the focus to certain areas of the frame, adjust for low or too much light, and a hundred other combinations of things.
If you're interested in learning about such things, the shortcourses website has a bunch of stuff. I bought a shortcourses book for my Olympus 5050 and learned a lot.
http://www.shortcourses.com/