Breaking the glass relieves stresses which otherwise would go into making molotov cocktail bonfires out of innocent people's cars. The masks are there to provide a face to wildly swing the mini-hammer at. Look how maddeningly smug looking they are!
You can tell they're thinking, "Hah hah I am a mask and not alive therefore I will smile as much as I want and you can't do anything about it!" Except you can. Heh.