Helmets do more than just protect....they hold microphones, earpiecses, breathing tubes, etc...They allow the pilot to stay concious, in control, and informed while flying...Kamikaze pilots still need to be concious in order to hit their targets
Simply put, the helmet was necessary to pilot the aircraft. Inside the helmet was the communication gear, and the goggles were attached to helmet as well.
This question is usually asked for its "joke" value, since it was a fact that the pilot was on a suicide mission. Rest assured, the helmet did absolutely no good in making the mission "survivable".
not all Kamikaze pilots were volunteers and many were coerced into the missions under pain of death...many Japanese people nowadays deplore the Kamikaze missions. The reasons above are correct for wearing the helmet but also, there were several Kamikaze survivors which can be googled...whether the helmets helped to save them is pretty dubious, though...