The manual mode is often referred to as the ultimate in creative photography. Why? Because manual mode allows you to use your intuition in order to set the aperture and shutter speed on your own. In other words, you are no longer dependent on the camera’s built-in metering system to dictate what exposure values you need to dial in (normal Auto mode).
The manual mode is necessary when you have to control both the amount of light as well as the depth of field of your shots. Let’s take a couple of examples. Let’s say that you want to shoot a seascape. You need to use a small aperture and then drag the shutter speed to produce the right exposure and composition. Similarly, if you need to use a wide open aperture in broad daylight in order to blur the background, something that the built-in meter will not allow, you can use an ND filter and open up the aperture wide to achieve that.