Most of your equipment in Helldivers 2 serves an in-game purpose. You need your weapons to spread Democracy and your Armor is there to protect you from enemy attacks. But, some items are there purely as cosmetics, ie, they offer no in-game benefits, they just look cool. Helmets are one of those purely cosmetic items that you can select before a mission, to complement your badass Armor.
Source: N4G PC Helldivers 2 Helmets, Listed