|Masks on American Horror Story, Coven|
Witches originally wore masks as a way of hiding their true identities. If anyone snuck up on them, they wouldn't be able to identify them. This would also include covering their hair.
As for the specific mask seen in American Horror Story, Coven, and its symbolism, here's a link to a Wikipedia entry for plague doctors, who wore beak-like masks thought to help protect them from air infected with plague as early as the 1600's.
|1656 engraving by Paul Furst of a plague doctor|
For the most part, when seen in television and movies, these masks just look cool and witchy!