Communication is a 2-way process. It is not simply about sending a message, but also about knowing if the receiver has understood and processed the message properly.
In face-to-face communication, the person listening may not always indicate verbally if they have understood or if they agree. Observing body language helps the speaker understand if the other person understands/agrees etc.
Also, even if the other person replies verbally his or her body language may show that they have not really understood or agreed.
Observation helps understand the effectiveness of communication.
This is because Body Language ( part of non-verbal communication) is instinctive and more reliable than verbal communication in many cases.
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