When a baby is born, it breathes naturally, its abdomen slowly rising and falling.
As we grow up, our breathing moves to our chest becoming ineffective and giving rise to tension.
We must re-learn to breath. We focus on doing abdominal breathing during our practices. All practices: yoga, meditation, martial arts begin with understanding and controlling the breath. (emphasize this).
Abdominal breathing must be practiced until it becomes our normal way of breathing all the time.
You will find dozens of articles and reasons to breath this way. A few of these are: