We tend to see body, breath, and mind separately, but in Zen Meditation they come together as one reality. Basically, zazen is the study of the self.
The great Master Dogen said, “To study the Buddha Way is to study the self, to study the self is to forget the self, and to forget the self is to be enlightened by the ten thousand things.”
To be enlightened by the ten thousand things is to recognize the unity of the self and the ten thousand things.
Upon his own enlightenment, Buddha was in seated meditation; Zen Meditation practice returns to the same seated meditation again and again. For two thousand five hundred years that meditation has continued, from generation to generation; it’s the most important thing that has been passed on.
Further information can be found here. Zen Meditation.