Accurate focus is important in any photograph, especially those with shallow depth of field. Depending on the photograph, focus and blur can be used to emphasize a subject. Blur can help eliminate distracting artifacts, while focus can stress areas of interest.
FIGURE 1: EXAMPLE: FOCUS ON EYES
Autofocus is a part of modern digital photography. It is a great tool because accurate focus can be determined much faster than a person manually focusing. This is important when photographing events such as weddings or sports.
For example, when taking a photo of person with a shallow depth of field, focus is commonly placed on the eyes. This is because we typically look into the eyes when we socialize with others. Placing focus on an ear doesn’t make much sense. I suppose unless explicitly photographing people’s ears, which I have not had a reason for.
Assuring focus accuracy must take place before clicking the shutter. It is not something that can be corrected afterwards in post processing. Today’s cameras and lenses all have autofocus.
HOW IT WORKS
Another Example: Phase Detection Interactive Example [standford.edu]
Autofocus is much more complicated than what it appears. The difficulties are more evident in low-light conditions. Luckily Nikon does a great job.