Why should Chinese medicine practitioners prescribe drugs by looking at tongue coating?

Tongue coating can reflect the nature of some diseases (such as white being cold and yellow being hot), the depth of some diseases (such as thick inside and thin outside), and the causes of some diseases, such as less red fur, yin deficiency, red tongue with heart fire, thick tongue coating with indigestion, thick damp heat and greasy, etc. , can give a preliminary diagnosis impression.