Planetside 2 has the option to purchase helmets in the game, but it is unclear if they actually help or if they are just cosmetic. Do helmets actually help by increasing armor, hitpoints, or reducing head damage?
You do not purchase helmets, you purchase skins for your helmet.
There is no stat effect from any of the skins for any piece of equipment or vehicle other than possibly helping you blend in to your surroundings when you are trying to be covert about your actions (white clothing on snow, green clothing on grass, etc.). This may help you not get noticed and therefore not be as likely to get shot at but it does not protect you in any way when someone is shooting at you.