How does perception occur in humans and what is the advantage of retention?
EDIT: Perception and retention are related to dharana (concentration of the mind) which is part of Astangik Yoga. It is mentioned in the Upanishads.
Simply retaining the breath alone is not pranayama. There are three parts to pranayama, restraining being only one. I read recently that there is a European man who holds what is considered the world record and can hold his breath underwater for 23 minutes. So at 3.5 minutes you have a long way to go.
According to Patanjali in his Yoga Aphorisms, the first step in yoga is Yama. Yama consists of 5 things. 1) Non-injury, 2) Truthfulness 3) Non-covetousness 4) Chastity 5) Sinlessness. The second step is Asana. Learning to have a correct posture while seated, firm, erect. The third step is Pranayama - suspending, restraining, and controlling the breath. The fourth step is Pratyahara - restraining the organs of sense from outward objects and directing them inward.
There are 3 other steps after this.
When trying to do Pranayama it should be done under the direction of your Guru. Doing it without your Guru's guidance can do more harm than good. It can lead to uncontrolled lust and your downfall.
The reason for Pranayama is that it is the 'flywheel' in the body to learn to control your prana. Once you learn pranayama correctly, you get hold of the thread that gets hold of the string that gets hold of the rope that controls the mind.
"Control the mind, cut off the senses, then you are a Yogi; after that, all the rest will come..." - Swami Vivekananda