I think you intend to know the purpose behind yoga. Its purpose is to achieve unification of the soul with the supreme soul to achieve divine powers, virtues from the Supreme Soul. To achieve such status, different disciplines of yoga have evolved as per the inclinations of the yogis, saints, rishis, etc. The purpose of yoga is to achieve health as defined by WHO - physical, mental, emotional, social and spiritual balance. If one aspect is lacking, the person cannot be called healthy (heal thy self), which can be achieved by self-realisation and god -realisation. So, the philosophy behind yoga is unification with self and god. Or we can say that the continued rememberance of Supreme Soul while being conscious of the self as soul, as Gita says.
