Our breath keeps us alive by refreshing our body with oxygenetated air and getting rid of the used, stale air, full of carbon dioxide. It has been said that the speed of our breathing can even determine our life span, the slower the breath the longer the life.
In Yoga we focus on the breath, breathing rhythmically and conciously, connecting to our breath, breathing more deeply, and even holding our breath sometimes. This focus on the breathing process helps us to stay in the now, not hooked on the past or worrying about the future.
The more primitive medulla oblongata in our brain stem determines our unconcious breathing but if we engage in conscious breathing we can signal the more sophisticated, evolved, cerebral cortex to liase with our hypothalamus for a more relaxing effect by soothing out our emotions.