Having an auto focus on your camera is definitely a huge benefit, but there are some occasions when manual focus may be the better way to go. Some of these instances include:
When there are many elements and details in the frame, it’s good to manually focus, especially if the subjects are close in shape, size, brightness or color. A good example is a field full of flowers and a street filled with people.
Subject is blocked
If the subject is blocked by an obstacle the manual focus should be able to help you focus on your main subject. If you use a wide aperture, you may also be able to blur the intruding object from view. A good example is a caged animal or somebody standing behind a gate.
Confusion in geometrics
When taking photos of architecture, the geometric patterns of a building can confuse auto focus on the camera, so you may need to manually do it.
If your image features a lot of sharply contrasting brightness it may also confuse the auto focus. The same problem can be caused by no contrast, especially if the subject blends in with the background. An example of this is a snowman in a field of snow.
If your subject is the smallest thing in the image, the auto focus will typically focus on the larger object. For example, if a person stands in front of a statue, the auto focus might pinpoint the statue.
If it’s extremely dark, it’s hard for the auto focus to see what you’re trying to shoot. If you want to capture falling stars or the northern lights, it’s a good idea to go manual.