Why is it necessary to scan entire body in Vipassana?

Mr Goenka teaches that there are six senses including mind, and that when we observe them in vipassana we eradicate our sankaras. My question is, nearly all my senses are on my face: like taste, smell, hearing, and seeing. Only "touch" and "mind" I am not sure of. So why not just focus on the my facial sensations, and do it smartly? Otherwise, what is the use of observing my legs' sensation, when I am fighting with anger, frustration, love, etc.?