From evolutionary perspective, such ability is crucial for our social functioning. It allows for recognising and avoiding threat; for establishing friendships/romantic relationships, etc. We can extract a hell lot of information about a person just by looking at their face:
3. Emotions/thoughts (through facial expressions)
4. Eye gaze - mutual attention
5. Lip reading
In this post I will discuss such topics as anatomy and neurology of face perception; facial expressions; innateness of face perception ability and deception.