Many other clues exist on your face. Most people can easily tell basic expressions like sadness or happiness from others like anger or surprise. The telltale signs of these expressions do not exist around your mouth like you would expect; instead they exist around your eyes and forehead. When someone is telling the truth, there will be lots of movement of the eyebrows making wrinkles in your forehead as opposed to when someone is lying they do not move.
Even though most expressions are universal, there are a few others which are completely culturally based. Those expressions were developed in a certain area and are not passed down by parents to children. In the western world a very rude expression is showing someone your middle finger. If you were to go to an African tribe, the native people from there would not look twice at you if they saw you making that sign unless they were told what it meant. Another rude hand sign that is completely cultural based is holding up your hand to someone as if to tell them to stop. In all the cultures of the world except African tribes, this has a universal meaning. To African tribes, however, this means the same thing as telling them that they have five fathers (one for each finger). Similarly in Iran if you were to give someone thumbs up, be prepared to have many people become angry at you because it means the same thing as giving them the finger.
The other reason is to express the inexpressible. When you are trying to explain something to a friend that is difficult to comprehend, you must use hand expressions. It is completely impossible to describe a color other than comparing it to another one, but with hand gestures, you can point to the color and easily show them what you are talking about.