The Aim of Yoga
According to the Yoga Sutras of Patanjali, the ultimate aim of Yoga is to reach Kaivalya (emancipation or ultimate freedom). This is the experience of one's Purusa (the innermost being or soul). Then one becomes free of the chains of karma (cause and effect) which tie us to continual reincarnation. In Kaivalya one is said to exist in peace and tranquility having attained absolute knowledge of the difference between the spiritual which is timeless, unchanging and free of sorrows, and the material which is not.
This is considered desirable as life is said to be full of sorrows and pain- even pleasure and joy leave pain and loss when they have gone as nothing in the material world is permanent.
Yoga is therefore a spiritual quest. However, along the path of yoga, the aspirant also gains health, happiness, tranquility and knowledge which are indicators of progress and an encouragement to continue their practice. Buddhism and other Eastern spiritual traditions use many techniques derived from Yoga.