Samurai masks offered a couple of advantages for their wearers in battle. Obviously, they protected the face from flying arrows or blades. They also helped to keep helmets seated firmly on the head during a fracas. This particular mask features a throat guard, useful for hindering decapitation. It seems likely that from time to time, as well, the masks concealed the true identity of a warrior (although the code of bushido required samurai to proudly proclaim their lineage).
The most important function of samurai masks, however, was simply to make the wearer appear fierce and intimidating. I for one would hesitate to cross swords with any samurai who showed up in this bristly wooden face-gear.
