Manual focus is there because auto focus systems have their limitations and can get confused ... They dont always work well or quickly enough. Sometimes they simply just dont work at all when the light is too dark. or too bright
Even when lighting is OK, Auto focus may not always know which part of a scene you want in sharp focus and if it uses a large aperture to expose the image, it will be so easy for the foreground or background to be out of focus with pleasing results
The Problem arises when it doesnt know within the scene which is foreground, main subject or background. You may end up shooting , say a wedding scene where the Bride and groom are OOF and the background or flowers are in sharp focus..

Having manual focus is a bonus when the amounts of contrast ( needed for AF) are not enough to rely on autofocus
Having a ring rather than a toggle is a more intuitive and "natural" way to achieve sharp focus manually
HTH