Every day you smile, roll your eyes, groan and perform all manner of strange behaviors in public with your body and voice. Yet you have managed to avoid a room in the loony bin. Of course, it helps that everyone else is doing those things too.
Non-verbal communication is made up of such familiar patterns of activity that we don't often think about their intended purposes. Your stubbed-big-toe yelp lets others around you know something's wrong. It's an evolutionarily useful tool, especially for the verbally challenged. Infants are famous for their fast recovery from scrapes and bruises to which no adult is witness, whereas their propensity for dramatizing a small but public boo-boo can cause much parental eye-rolling. But even adults want attention when hurt.
One 2003 study observed that, for some adults (labeled "high catastrophizers"), pained expressions and vocalizations lasted longer when observers were present. This suggests that one purpose of vocalization may be to communicate dependency needs and encourage communal coping.