Coloring helps the child to learn how to control the scale of his hand. Please note that when the baby is just started to draw he does not monitor the boundaries of the sheet, and the picture is not painted on the outlines, and even beyond.
There is another advantage of colorings for adults. Most of them are very nice and include impressive black-and-white contours that can be turned into a work of art. Imagine the feeling of satisfaction that you get when you have finished drawing and will be able to appreciate the beauty of the masterpiece created by you personally.