Because through body language, people can show their attitude in what they're saying(very excited bored...) and therefore, affecting the overall passion in the speech.
Moreover, body languages and hand gestures are used to describe what can't be described with languag.
Also, people are more affect by what they see more than what they hear and therefore, would prove to be a better method of understanding (especially through language barriers)
Goachakorn 005 3EN