Just so simple;
Breathing involuntarily is controlled by our Medulla Oblongata. A gland at the base of our skill that sends signals to our heart & lungs to speed up or slow down when the CO2 level gets too high in our body. It triggers us to draw in more O2 which increases the capacity of our muscles etc to help with the fight or flight scenario. Breathing Voluntarily, through pranayama and breath awareness is controlled by our Cerebral Cortex, grey matter.
In a simple nutshell; Breathing voluntarily gets the brain focus on drawing in and out the O2 and Co2 and so preventing the mind from wandering off to plans, expectations etc.
Taking the time to control our breathing helps to calm the mind, ease stress, balance emotions and bring about equilibrium.
So learn to just breath and get back into balance!!