Your DSLR has comes with a highly advanced autofocus system, so why on earth would you want to use manual focus? Actually there are some very good reasons - various subjects and environmental conditions either fool the camera, or make it considerably harder to get a good shot in autofocus mode.
The AF sensor in your digital camera needs certain things to perform well, and at the top of the list is light and contrast. It uses edges or textures to focus on areas of contrast. If you're shooting in low-light, AF can have problems seeing subtle, indistinct features.
If contrast is low, as in misty conditions or when aiming at smooth water or wet sand, your camera's AF circuitry has difficulty locking onto the subject. Manual focus, then, helps you get sharp shots when AF can't correctly interpret what the lens is seeing.
Another set of AF issues occurs when the camera focuses on the wrong thing. Shooting through a wire fence or a glass window, for example, can cause focusing problems in AF mode because the camera focuses on the obstruction, rather than the subject beyond it, and likewise it can be particularly challenging to get good nature shots through branches, leaves or long grass.
Your camera will always focus on the closest thing the sensor sees, and this can cause problems when shooting a particular animal, when another one flies or walks through the frame and distracts the AF.
So switch to manual focus when you know your camera could get confused.
Get close to your subject
Although most macro lenses have autofocus, manual focus is the best technique for close-ups of small subjects.
You'll quickly get frustrated trying to autofocus on a butterfly or bee because even the slightest movement, by you or your subject, will cause the lens to go off on a slow hunt for focus - and by that time the subject is long gone!
The final set of conditions where you're better off focusing manually is to do with speed: either because the subject is moving so quickly that it's hard for the camera to focus in time, or the slight delay of hunting to achieve focus is long enough to miss the shot.
When shooting racing cars on a bend, for instance, it's often better to pre-focus on a particular spot on the track in AF, then lock your focus in MF, and wait for a fast-moving car to reach that spot before taking your shot.
The same principle applies in nature photography, where pre-focusing on a perch allows you to prepare for a bird's landing or take off to get a crisp action shot without AF delay, or even when photographing your children in the park.