Great question, Rota! The answer is "education."

Education is the learning of a new concept, skill, or idea. The vehicle for learning is social interaction with a peer. Sometimes that peer is less able than ourselves (me assisting a forum n00bie) or equal (Goats reminding me how to search for spambots), sometimes the peer is more capable (H1T teaching me how to make sigs), and sometimes the mediation occurs through individual interaction with a text.

I would argue that essentially the function of communication is learning. Fall's cave paintings, Rota's sanitation, and Conrad's philosophy nominations are all instantiations of this, humankind's greatest achievement.