This can be used to rotate the image for an interesting effect, or more commonly to correct the image if it is at a slight angle.
If your photo is too bright or dark, you can adjust the brightness in order to get a well-balanced image.
Adjusting the contrast affects the difference between light and dark areas of the image. You can fine tune it to meet your needs, such as increasing the contrast to make your subject stand out more.
With manual white balance you can choose the most accurate setting and avoid the green, blue or red hue which can occur in auto mode. You can also change the setting for artistic effect.
Saturation refers to the intensity of colors. It is measured on a scale ranging from 0% (black and white) to 100% (pure colors) at a constant level of brightness. Images with high levels of saturation have vivid colors, while unsaturated images appear greyish.
Using selective adjustment points, you can change the settings (such as saturation, brightness, etc.) of a particular part of the image. This is useful if you wish to edit certain areas without changing the whole photo - for instance, if you wish to increase the color intensity of an object in the image.