It filters his voice, Kakshi kill his mother when he was borned because of his thunderous crys and his unique abilities to shoot lighting from his mouth, zatch bell style. He never really learned to fully control it, he can only surpress hid voice and lighting blast for about five minutes straight. . So he wears it to filler his voice and supress his lighting to protect others. He wouldve killed zabuza no diff if naruto sakura and saskue wasnt there. When he kissed that firl in the filler why do u think her eyes widened? Because his kisses are powerful and shocking