Hi, i am no scientist but i remember reading somewhere or other that it harks back to our evolutionary past; when a group member had imbibed something harmful they would vomit to remove it from their digestive system so as to minimise it's harmful effects - the other group members (who likely consumed the same substance) would also vomit as a preventative response (thus removing the offending matter even before a small amount is digested to elicit the 'true' vomit reflex, again reducing exposure and maximising survival).