In this mode, you can manually set both the aperture and the shutter speed to any value you want – the camera lets you fully take over the exposure controls giving you greater flexibility to set your shots up exactly as you wish.
This mode is generally used in situations, where the camera has a hard time figuring out the correct exposure in extreme lighting situations. For example, if you are photographing a scene with a very bright area, the camera might incorrectly guess the exposure and either overexpose or underexpose the rest of the image. In those cases, you can set your camera to manual mode, then evaluate the amount of light in darker and brighter areas and override the exposure with your own settings.
Manual mode is also useful for consistency, if you need to make sure that both shutter speed and aperture stay the same across multiple exposures. For example, to properly stitch a panorama, all shots that you are trying to put together need to have the same shutter speed and aperture. Otherwise, some images will be darker, while others are lighter. Once you set the shutter speed and aperture to the values of your choice in manual mode, your images will all have consistent exposures.