Yoga is definitely a very beneficial exercise. However the purpose of yoga has disappeared in modern times. Indian yoga which has a highly spiritual nature has degenerated into a form of 'mental and physical exercise'.
Pls see the hindu website:
Modern Yoga versus Ancient YogaThe goal of Yoga is Yoga: The goal or destination of Yoga is Yoga itself, union itself, of the little self and the True Self, a process of awakening to the preexisting union that is called Yoga. While it is not the intent of this article to give a final or conclusive definition of the term Yoga--which can be described in different ways--it has to do with the realization through direct experience of the preexisting union between Atman and Brahman, Jivatman and Paramatman, and Shiva and Shakti, or the realization of Purusha standing alone as separate from Prakriti. The mere fact that one might do a few stretches with the physical body does not in itself mean that one is headed towards that high union, referred to as Yoga.
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Perception has recently shifted: The typical perception of Yoga has shifted a great deal in the past century, particularly the past couple decades. Most of this is due to changes made in the West, particularly in the United States, though it is not solely an American phenomenon. (Similar shifts have happened with Tantra as well.)
Gist of the two perspectives: The gist of the shift can be summarized in two perspectives, one of which is modern and false, and the other of which is ancient and true.
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False: Yoga is a physical system with a spiritual component.
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True: Yoga is a spiritual system with a physical component.
The false view spreads: Unfortunately, the view that Yoga is a physical exercise program is the dominant viewpoint. The false view then spreads through many institutions, classes, teachers, books, magazines, and millions of students of modern Yoga, who have little or no knowledge or interest in the spiritual goals of ancient, authentic Yoga and Yoga Meditation.
pls read entire article in that website
Tibetan Buddhism yoga however, is fully for spiritual purposes.