If it is the same as the RAF, the helmet contained the oxygen mask and radio. They needed to pilot the plane to get to their target.
The cloth helmets they wore contained the ear pieces for their radios and goggles for eye protection. Until the post war era hard hat helmet for aviators were uncommon. Oddly, the russians late in WW2 were the first to issue a hard shelled helmet to fighter pilots. US pilots followed. By 1950 they were in common use.
They still needed to reach their targets, the helmets would help protect them until that point.
the Japanese were into health and safety before anyone else had even heard of it or as David G.H says.
It is a protection for their brain if they accidentally bump their head on the ceiling of the plane.
for the same reason they wore clothes. it's part of the uniform.
The only unsolved mystery so far, why would they wear helmet when they are about to commit suicide. It just doesn't make sense
