Deep breathing is beneficial to the gas exchange between the body and the outside world. Deep breathing in qigong is a deep and slow type, which can discharge the waste gas that can't be excreted at ordinary times.
Body fluid is transformed from kidney essence, and the tongue touches the palate, which will connect the two channels of Ren Du and accelerate saliva secretion. After swallowing saliva, nourish kidney essence.
I am a freshman in College of Traditional Chinese Medicine, and that's all I know. In fact, why practicing Qigong is good is still an unsolved mystery. If you have to explain, you can only make it up like me.