Face recognition is basic, hard-wired into us from birth. Newborns recognize a face versus a drawing of a not-face and learn their mothers' faces within a few hours of birth.

Any child (barring limitations of disability) can instantly recognize family and friends, can identify emotional states from facial expressions and, early in life, is able to draw the semblance of a face with the salient features correctly positioned.

We can tell at a glance, for most people, whether the face being viewed is a man's or a woman's (more gracile, heavier-browed, 5 o'clock shadow, etc.).

Identification documents (such as passports and licenses) require full face photos, with (perhaps) other identifying features.

When introduced to a new person, we look at his/her face. How many times have you heard, "I can remember the face but I've forgotten the name"?

Deformed facial features are especially repugnant to us, more so than loss of limb or body scar, because our face is so closely linked to our identity. That's why horror movies often have a protagonist with twisted facial features or who wears a mask.

Such is the importance of faces to us.

He or she who has an obscured face (ski mask, veil) is associated with a crime or other state of non-being, sub-human. To be forced to live veiled is dehumanizing.

Which is, I guess, the whole point.

Note: Barbara's link to The Guardian article in her comment is quite interesting.