Pranayama exercises are supposed to purify the nadis (subtle energy channel, equivalent to a meridian in acupuncture), not just your nerves. Incidentally, these are all side benefits.
When you are doing your yoga postures you are being inwardly awakened, but your body also benefits. I would call it a fringe benefit, not the real benefit. In the same way, when you do the pranayama exercises, the mind becomes steady and the nadis become purified. That the nerves and mind are also calmed is a fringe benefit.
Pranayama exercises have a tremendous meaning. Therefore one who does pranayama or breathing exercises as part of yoga has to approach it in a different way – not merely use these as breathing exercises, but as a part of an inner adventure. By the practice of pranayama you come face-to-face with the power we call prana. You know that prana. It is not possible for anyone to show you, but within you you will know what it is – Swami Venkatesananda