Lets put it this way: As with several other features of the human body, science has not yet found the evolutionary reason.
It is not that long ago when scientists wondered about the existence of the appendix. Before antibiotics and surgery, an estimated 5-10% of the population died of appendicitis, so there was an immense evolutionary pressure to get rid of it. But it was still there, leaving scientist baffled until they found out how important the appendix is for survival.