Cinematography, Lighting, and Composition
The reason manual controls are critical is because the images that move in front of the camera vary in color and light reflectance, depending on how dominant they are in the frame. An automatic camera is programmed to constantly adjust and average for the best exposure for each individual frame. This is very convenient for still photography, where each photograph exists on its own when it is viewed. In animation, we often look at hundreds of images together; and if the automatic exposures are changing from one frame to the next, as the imagery changes in front of the camera, then there are wild fluctuations in the exposure as a movie plays. This is highly distracting. Controlling the exposure, white balance, focus, and image stabilizer and locking them in with a manual setting helps to eliminate this fluctuation.