Why is it important to observe an individuals reactions when communicating with them

To view their body language, it's a huge part of spoken language as it can define what type of speech it is. For example if they have their arms close in they may not want to speak to you or if they are doing lots of hand gestures they are explaining something. 

It is important to note how people react for different reasons. the biggest reason is to see emotions that people do not convey through speech. People constantly show what mood they are in by body posture. Also, paying attention to reactions will also help in both remembering what is being told to you, as well as show someone you are interested in what they are saying.