British WWI tank crews were issued these masks, they were to protect the face from metal splinters coming from the inside face of the armour when hit by enemy fire. Even machine gun bullets which failed to penetrate the armour could cause splinters on the inside.
Snipers or marksmen would reverse the head of a bullet effectively making it a Hollow charge , they wouldn't penetrate the plate of a Tank but knock a chunks and splinters off the inside of the tanks compartment just as 'aghart' mentioned . a practice still carried out today on lightly armoured vehicles in Afghanistan .
The link shows the BBC programme called Soldiers, a history of men in battle. Episode 6 "Tank". It shows the tank mask in use. The programme was made in 1984 and aired on BBC1 in 1985. I was one of the soldiers who crewed the WWI tank at the tank Museum at Bovington. That's me The whole crew
Watch the whole link, the last minute shows splinters coming off the inside armour and the mask protecting the gunner.
A neighbour some years ago a retired Squadron Leader was a pilot on Vickers Virginias at Worthy Down, he showed us his chain mail and leather face mask, 'flying in an open cockpit when hail hits was not much fun'. The birth of the Tank parts 1 to 5: http://www.kingscollections.org/servingsoldier/collection/the-birth-of-the-tank-part-1
Tanks: The log book of a pioneer 1948-1918: http://www.scribd.com/doc/43908875/Tanks-1914-1918-the-Log-Book-of-a-Pionner-UK-1919
