Breathing plays a significant role on the yogic path, in that it is both a lifestyle decision ("I will breathe properly throughout the day") and a set practice ("I will practice certain breathing techniques for a certain amount of time every day"). Some yoga practices like postures primarily affect the body, while others like concentration and meditation primarily affect the mind. But breathing plays such a significant role on the path because it uses the lungs of the body to positively affect the centeredness and presence of the mind. When we breathe properly, we are less affected by our emotions and are able to conduct ourselves with presence and focus.
But, the flip-side is also true. Most of us are slaves to improper breathing, for while a deliberate breathing routine leads to peace, an indulgence in conflict through arguments and violent films leads to improper breathing. For those of us who have fallen into a habit of improper breathing, it is only when we make a conscious effort to live more peacefully through a path like Yoga that we have a more beneficial lifestyle. And, with conscious effort comes a proper habit.