Imagine someone speaking to you with a blank face. This person has no expressions. How would you feel? Will you understand everything this person says? Wouldn’t it be better if you see happiness on the face when you hear something happy, or see an expression of sorrow when something sad is being spoken about?
Your facial expressions help the audience as most people are visual in nature. We pick up meanings easily through the facial expressions. In fact, if you face speaks a different language than your words, people would be confused, and in fact, would be more likely to believe your face than your words. For example, you show a sad face and say you are happy. What would the audience believe?