Nonverbal communication incorporates eye contact, gestures, facial expressions, posture, frequency of glances, even blink rate. It is a an important part of communication.
A UCLA research has shown that only 7 percent of communication is based on the actual words we say. As for the rest, 38 percent comes from tone of voice and the remaining 55 percent comes from body language. Learning how to become aware of and to interpret that 55 percent can give you an advantage with other people.
Part of my teaching includes practice with intonation of speech, projecting voice using good posture and oral-nasal resonance techniques. All of these parts of speech are important and convey confidence and competence to your listener.