Is the fantastic impracticality of these helmets due to the handicap principal ( https://en.m.wikipedia.org/wiki/Handicap_principle )?
Some of these are almost certainly just ceremonial pieces, but the ones that aren't would have served a distinctive signpost for troops under a samurai's command, the same way that flags strapped to armor and horses were common in premodern warfare through the world. Fighting without radios or even so much as binoculars is extremely confusing, and if you're leading a bunch of peasant militia who get a bare minimum of…
Side note : one of the most amazing parts of the exhibition was the archery equipment -- fantastic and frightening-looking arrow broadheads especially