Some masks are meant to conceal, as with bank robbers, say, or dancers at a Venetian ball. People are more daring when their features are obscured.
Then there are death masks, using casts made of mates or children or historic figures, such as Abraham Lincoln, after they have taken their last breath. (Lincoln actually had a mask made before his assassination, and another was made after, according to Wikipedia.) The Victorians were fond of these masks, it seems. Phrenologists, too. They thought bumps on the head revealed character traits.