Cecil got it mostly right, but not quite.
First, he says that fighters taxiied with canopies open for customary reasons, or for reasons he didn’t understand. The actual reason for taxiing with the canopy open was so that the pilot could crane his head out of the cockpit to see where he was going. WWII fighters almost universally had big, long noses and a tailwheel for steering. As a result, visibility straight ahead when taxiing was zero. Pilots overcame this by taxiing in a series of S-turns, and by sticking their head out of the window.
Anyway, the purpose of the helmet was to keep your radio headphones on, and to keep your head warm. Fighter planes in a dogfight (or in turbulence, or when manoevering to dive on the enemy) could pull many G’s. At 6 G’s, those one pound headsets weigh six pounds, and are trying frantically to pull themselves off of your head. Those leather helmets actually had the earphones sewn right into the sides.
Keeping warm was very important, since these fighters routinely cruised at altitudes of 20,000 feet or above, where it might be -30 outside. Heaters in those fighters didn’t work very well. I’m not even sure the Mitsubishi Zero even had a heater. If you ever see pictures of those Kamikazi guys, you’ll notice that their helmets are usually lined with fleece, and they are wearing heavy jackets even though they are launching from the tropics.