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Why breathing should be mainly diaphragmatic at rest
Apart from several other effects, there are 2 key reasons why diaphragmatic breathing is superior to chest breathing.
Homogenous alveolar gas exchange
People with heavy or deep breathing are often chest-breathers since the smooth muscle of the diaphragm is in the state of spasm due to CO2 deficiency. Hence, their lower layers of the lungs get much less, if any, fresh air leading to reduced oxygenation of arterial blood in the lungs due to so called "ventilation-perfusion" mismatch. Normal breathing is diaphragmatic allowing homogeneous inflation of the whole lungs with fresh air, similar to what happens in the cylinder of a car due to the movement of the piston. Hence, during diaphragmatic breathing all alveoli are homogeneously stretched vertically and get air with higher O2 concentration.
Efficient and constant drainage of the lymphatic system
The lymph system, unlike the cardiovascular system with the heart, has no pump. Lymph nodes are located in the places of the human body that get naturally compressed (squeezing) due to movements of, for example, body parts, as in cases of lymph nodes located around the neck, above arm pits and groin area. However, the lymph nodes from stomach, kidneys, liver, pancreas, spleen, large and small colons, and other vital organs are located just under the diaphragm. This is because the nature expects us to use the diaphragm in order to remove waste products from these vital organs all the time - literally with each breath. Hence, chest breathing causes stagnation in the lymph system and accumulation of waste products in vital organs located under the diaphragm.
For the list of the quoted references click here
(Excerpts from the book "Normal breathing: the key to vital health")