A wild guess, though !! Since, the tail exists with human fetus during the first 8 weeks of pregnancy, might be having a specific function unknown presently, which medical experts can findout somtime in future !! It definitely must have some purpose.
Look, people wondered why un-necessary items like tonsil and apenix exist in human body, but these acts as indicators of initial infection of throat and intestines respectively.so that early corrective action taken to check deepening of infection
