Tripods are used for both motion and still photography to prevent camera movement and provide stability. They are especially necessary when slow-speed exposures are being made, or when telephoto lenses are used, as any camera movement while the shutter is open will produce a blurred image. In the same vein, they reduce camera shake, and thus are instrumental in achieving maximum sharpness. A tripod is also helpful in achieving precise framing of the image, or when more than one image is being made of the same scene, for example when bracketing the exposure. Use of a tripod may also allow for a more thoughtful approach to photography.
For all of these reasons a tripod of some sort is often necessary for professional photography. In relation to film/video, use of the tripod offers stability within a shot as well as certain desired heights.