[crooked-v]

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 training, you want it extremely obvious who's giving the orders and where to try and group up if people get separated in battle.

[Isamu]

Yes. Important and high-level people often wore large and conspicuous helmets to show that they were still on the field. Others could look to see that the lord was still not overrun or had not taken flight, or perhaps he had taken a different position. Also it could be useful to signal his authority if he had to directly rally troops from horseback.

[MisterOctober]

Yep! I went to the "Art of Armor" exhibition when it came to Phoenix Art Museum - saw this stuff up close and bought the catalog, which confirms that many of the fancier pieces were ceremonial in nature -- especially the ones with big vertical decorations

Side note : one of the most amazing parts of the exhibition was the archery equipment -- fantastic and frightening-looking arrow broadheads especially

[sorokod]

...and the face masks?

[jschwartzi]

These would be useful for both terrorizing your enemies and also ensuring that you don't take a blow to the face.

[crooked-v]

Also, easily exploitable high-quality iron was much rarer in Japan than in Europe, so reinforced masks gave some face protection while being incredibly cheaper than steel faceplates on helmets.