Why we should pay attention to body language in business?

Body language is quite important in business,as we all know,we cannot judge a person totally through his speech,we need to notice his/her body gestures,facial impressions etc to see his/her thoughts.For example,if he/she keep looking at your eyes directly and keeps nodding head,that means he/she agree with what you said,however,if he/she moves aways his/her eyesight and keeps looking at somewhere else,that indicates that he/she may not be interested with what you said at all.And when we talk with others,we should be polite,sometims using body language,such as nodding,leaning forward a little to show interest etc.