Drawing the face is the most crucial point to any illustration. The face holds the capacity for expression, and therefore reveals the character's personality, thoughts, and desires.
I like to think of the face as the house of the character's soul. There is so much about an anime or manga character that you can show through the face.
For example, a scar on the face suggests that this character is battle-worn and a warrior. On the other hand, big, glistening eyes and unblemished skin imply innocence and youth.