It supplements words in the expression of feelings. To lose it is like losing part of one's civility. The use of eye contact is also culture dependent. Americans may find it uncomfortable to talk to their Japanese partners, who often do not maintain eye contact. A Japanese would be horrified to see an American driver shift his gaze away from the traffic to talk to the passenger.
Smooth communication takes more than the knowledge of language and manners.
The pace of communication also differs between America and Japan. Often Americans talk incessantly. But Japanese proceed more leisurely, pausing now and then to assess the situation or to let the silence speak. When English is the language of choice, the American's tendency to talk without pause can easily overwhelm the Japanese, whose English proficiency is most likely not high enough to listen and simultaneously think of what to say next. If his American partner is not sensitive enough to the situation the Japanese would be forced to choose between listening and talking.