We use the act of breathing as prop because it is bridge between the internal and the external. Except the breath, there is nothing else, which comes out as well as goes in.
If we desire our mind to stop wandering and to undertake an internal trip, the easiest way is to make it ride the chariot of breath and obtain access to the inside with it. With the commencement of the internal trip, one will become an introvert and a spiritualist. Thus association of the mind with the breath is the easiest way to become spiritual.