Why does it help to focus on the breath when practicing mindfulness?
The art of meditation, which is another name for the practice of mindfulness, is all about being in the present.  In other words, cultivating an awareness of what is happening in the present moment.  This is done by quietening the mind of all the random thoughts that are being continuously generated, and having the senses alerted to the signals that are being passed in the moment.  The signals are the sensations or emotions or sounds that one is feeling and hearing in the present moment.  To accomplish this, the breath serves as an anchor to the present moment.  Being aware of one's breath keeps us in the present moment.  When the mind eventually wanders, the breath is a signal to tune back into the present moment.  The breath is always with us and can be found in an instant.