Banes face is most likely extremely disfigured also. Thats what he means when he says the mask contains his pain. Mayb his pain is bc he was disfigured n his face is shattered. He means that the person will b disgustd horrifyd n shokd when they look upon whats left of his face. It protects his shatterd features. Also allows to breathe in a painkiller contained n distrubuted in the mask while securing facial muscle integrity