In the practice of yoga asana we synchronize breath with body movements. Why is that?
Because for most people the only way to access the internal stuff is through breath. Advanced yogi can manipulate their internal body functions by directly manipulating their prana, but common people can only control the prana through breath.
In yoga asana practice, we practice focusing on breath. Its like practicing holding a rein.
The body and mind is like two wild horses attached to a chariot. On the chariot is the driver, the soul or consciousness. Most of the time the driver is asleep; so the two horses run around without direction and do whatever they want. And when the driver wakes up, he didn’t know how he get there!
Breath is like reins for the horses. With reins the driver can control the horses. But first the driver have to learn how to use the reins. When we are familiar with the reins, we can sense the horses moves through the rein and we can start working on taming them until finally they can move in sync and can stand still when told to.
When we are fluent with breath, we became more aware and more in control of the body and the mind. That is why in yoga asana we practice with breath. But first the consciousness has to wake up and start practice holding the reins. We have to become fluent and intimate with breath.