The purpose of yoga
"In the western parts of the world, if you utter
the word "Yoga," people think of impossible physical
postures. This is a very distorted idea of what Yoga
is. Yoga is not about bending and twisting your
body or holding your breath. It is a mechanism and
a technology to get you to that state of experience
where you see reality just the way it is.
The word "Yoga" literally means 'union'." -Sadhguru
The origin
"In the yogic culture, Shiva is not known as a god, but as the Adiyogi or the first yogi - the originator of yoga. He was the one who first put this seed into the human mind. According to the yogic lore, over fifteen thousand years ago, Shiva attained to his full enlightenment and abandoned himself in an intense ecstatic dance upon the Himalayas. When his ecstasy allowed him some movement, he danced wildly. When it became beyond movement, he became utterly still."
Why?
"Yoga means to liberate yourself from that information which determines who you are right now. That information which determines the color of your skin and the shape of your body should not determine how you think, feel, and experience your life. If that information does not determine how you are right now, then you are moving into Yoga."