All helmets had a built-in air filtration system, which protected the wearer from dust, ash and so on. Benjamin Carmine mentions the dust filter, and Jacinto's Remnant mentions the helmets protecting Gears from breathing in the ash following the world-wide hammer strike. Plus, the helmet is very protective, this is proven by Clayton Carmine's life being saved when a bullet from a stranded's sniper rifle shot hit the side of Clay's helmet, but the bullet was deflected and Clay's head stayed in one piece.
However, they have disadvantages. They reduced the perception of the wearer, making them vulnerable to threats such as snipers. Anthony Carmine demonstrated this vulnerability when he was killed by a sniper he had not seen.