There has been a lot of controversy regarding head guards in boxing in recent years. Recently AIBA has banned their use for males in international competitions (not for females though :S).
Head guards are intended to prevent serious head injuries whilst sparring or competing, and also to prevent lacerations. However several studies, including one commissioned by the association, found that the number of acute brain injuries declined when head guards were not used.
This was found to be due to two reasons. Firstly, head guards make the target area much larger. It becomes easier to hit and encourages the opponent to hit the head more often. In addition it gives the boxer a false sense of security. However they are good at preventing lacerations to the face.
Boxing SA stipulates that boxers MUST wear them during sparring or they will not be covered by their insurance. For this reason we enforce wearing head guards in Dwyer's Boxing Club. But it does beg the question whether their policies will be updated.