Breathing is the primary bodily function that is both autonomic (automatic, like the heartbeat), and voluntary. The only other one I can think of off the top of my head is blinking your eyes. Yogis have taken this to mean that the breath is the road to greater control of autonomic functions of the body (among many other things). Given some of the performances skilled yogis have given (like slowing their own heartrates and such), they may just be on to something.
There is a great book by Swami Rama called "Science of Breath" that goes into breathing in great detail, from both a western scientific view and an eastern religious view. A very interesting read.
My personal opinion: breathing is a great joy, when given proper attention.
The Rev