The purpose of pranayama, or mystic yoga, is to stop the mind and senses from engaging in fruitive activities. The so-called yogis who practice in the Western countries have no idea of this. The aim of pranayama is not to make the body strong and fit for working hard. The aim is worship of Krishna. The minds of those who are addicted to fruitive activity are always filled with unclean desires.
Fruitive activities are symptomatic of our polluted desire to dominate material nature.
As long as one continues to be subject to polluted desires, he has to accept one material body after another. So-called yogis, without knowledge of the real purpose of yoga, practice it in order to keep the body fit. Thus they engage themselves in fruitive activities, and thus they are bound by desire to accept another body. They are not aware that the ultimate goal of life is to approach Krishna. In order to save such yogis from wandering throughout various species of life, the sastras warn that in this age of Kali-yuga such yogic practice is simply a waste of time.