Some instructors are also inhibited teaching pranayama because they believe it has some psychic and mystical value(it has that also). But for a beginner in yoga, the impact is largely biological.
Below is why:
According to the well-researched book Freedom from Stress by Dr Phil Nurenberger the nasal passages regulate the pressure of air entering and leaving the lungs. This can be measured by the Cottle rhino meter.
This pressure sets off reflexes in other parts of the body, including the brain and the autonomous nervous system. This in turn can have important psychological and physiological impact.
For one, the nasal lining has a heavy cluster of nerve endings. It has erectile tissue similar to that found in the genital region, which manipulates the flow of breath which in turn affects the inner homeostasis. The roof of the nasal cavity is closest to the master gland pituitary which is constantly managing the homeostat of our internal biological functions like temperature, blood pressure, heart rate, breath rate, blood circulation etc.
And since it is also closest to the olfactory nerve, the nasal air flow is also linked to the most primitive part of our limbic system that strongly affect our subconscious and emotional impulses.
Now u know why I love pranayama:)