Yoga means union, to be one with one's higher self. It is a Scientific Art to Self-Realisation. Yoga has been tried and tested for centuries by the Yogis of India. The purpose is to find inner peace, happiness, good health and ultimately Self-Realisation. The first steps of yoga involve postures or asanas. Other forms of yoga involve mantra, bhakti (devotion), karma (work), pranayama (breathing) and dhyana (meditation).
There are many postures that have a related health benefit. However, it's main purpose is to strengthen the body and nervous system in preparation for the higher forms of yoga such as Kundalini Kriya Yoga. The body has to be able to cope with the high quantities of energy that is generated from the pranayama kriyas. The body must also be strong to sit in meditation for long periods of time.
To be able to realise the Self, requires the mind to be still. Doing postures, breathing and meditating requires serious effort, concentration and will power. Yoga is hard work, however, the benefits of Yoga have to be experienced! Anyone who tells you "it is easy. You just sit there", has not a clue.
So, do Yoga and achieve good health, happiness, peace and know your true Self!
Try it. The proof is in the experience!