Breathing is very essential for living it is what that keeps us alive. Pranayama is the process of elongation of breath. Pranayama is a series of breathing technique where the breath is controlled along with various hand positions called mudra which control the flow of prana or Life Force Energy to various parts of the body.
Let us consider an example animals like tortoise which takes about three breaths per minute live to about 180 years the secret to this is the way it breathes. The same way elongating the breath increases the quantum of prana or Life Force Energy that is absorbed into the body during each breath.
The benefits of Pranayama are
It boosts the immune system
It increases the quantum of prana in ones body
It revitalizes the body and mind
It cleanses the body of toxins
It activates the nadis or energy meridians
It relaxes the mind and cleanses it