Why you should listen?

Answer 1

You should not only listen, it must be done actively too.

This is because if you don't listen properly, you may miss significant points in a conversation. It's only when you understand the message that you can respond in the best way to whatever is being communicated.

It is true that communication is complete when the encoder of information receives a feedback from the decoder.

This simply reiterates the fact that if we do not listen actively, we may miss some salient points of the communication and end up giving erroneous, incomplete or biased feedbacks to issues of great importance.

Answer 2

... because it displays respect.