The Grayscale tonal chart and histogram are important in photography for many reasons, one of which being, so all the colors come out true in detail and tone for the shot. The histogram translates how the zones look in a graphical representation so you'll know whether you have the proper exposure or not. Being able to have control over these gives you the ability to assure that your fabrics such as black leather will appear to be black leather rather than just a dull black with no texture or depth. It also assures all the tones are exposed properly. Being able to adjust this will generally give you a little bit more control over your environment to achieve your desired results.
It's also very important to capture the correct tones because you cannot make or change something that truly wasn't there to begin with. Correction can only go so far and still be natural looking. Every time you alter a picture in Photoshop you're damaging the photo and destroying pixels. It's best to do as little as possible. I would also suspect in the digital world it would also help in keeping the files smaller.
Using depth of field in pictures such as portraits is very important to draw your eye to the main subject, would be which would be the people in the portrait in that instance. Having the ability to blur out the background will make them stand out more and keep your eye on them. Depth of field is also great for shoots for magazine ads like clothing, sophisticated backgrounds can still be used but the focal point, which is whatever clients are selling or representing, can be kept in focus in the background or foreground to draw the attention to it. There's endless possibilities when shooting varying shots with different depths of field.
Obviously one of the best uses for shutter speed would be for action sports photography. Using the appropriate shutter speed you can stop movement of an object with perfection and clarity. For instance you could shoot a swimmer at a...