The REAL essence and power of mindfulness is becoming aware of the contents of your attention. For some reason, focusing the attention inwards on bodily somatosensory experiences tends to encourage that, but the two are not the same. Body-scanning is more a technique to help encourage the development of mindfulness rather than the end goal in itself.
The reason this distinction is so important and powerful is that the brain regions which are feeding the contents of your attention are the ones that get reinforced. When you combine mindfulness with practice in redirecting your attention, it becomes an insanely powerful tool to fundamentally reshape your reality by restructuring your brain.