just a short answer, since no-one has responded yet:
Except from deepening your breath by expanding the volume of the lungs down towards the belly,
the diaphragm is also involved in non-respiratory functions, helping to expel vomit, feces, and urine from the body by increasing intra-abdominal pressure, and preventing acid reflux by exerting pressure on the esophagus as it passes through the esophageal hiatus.
But this is only one aspect of many more, others are
- posture -> since the diaphragm inserts at the lower ribs and the vertebrae it influences the way you spine is aligned
- lung volume -> when the diaphragm contracts during inhalation, it expands downwards and gives way to the lungs to expand as well. thus you have a bigger volume and can breath in more air.
- slower breathing -> the more air you breath in, the slower you breath, this may lead to a relaxation during stress situations.
It is often considered superior to mouth breathing for several reasons. Air travels to and from the external environment and the lungs through the nasal passages, as opposed to the mouth. The nasal passages do a better job of filtering the air as it enters the lungs. In addition, the smaller diameter of the nasal passages creates pressure in the lungs during exhalation, allowing the lungs to have more time to extract oxygen from them. When there is proper oxygen-carbon dioxide exchange, the blood will maintain a balanced pH. If carbon dioxide is lost too quickly, as in mouth breathing, oxygen absorption decreases. Nasal breathing is especially important in certain situations such as dehydration, cold weather, laryngitis, and when the throat is sore or dry because it does not dry the throat as much.
From wikipedia as well.