The Yamas and Niyamas are self-disciplinary qualities that everyone should have and observe for their own spiritual development. One may practice Asanas and have a fit body. One may practice Pranayama and balance the Pranic energy, the Nadis. One may practice Pratyahara and Dhyana and reach deeper state of consciousness, but what use is that if one does not practice the Yamas and Niyamas. The Yamas and Niyamas create a fit and balanced mind.
Yoga has a deep hidden philosophy of life that harmonizes body, mind, and soul. The term was originated from a Sanskrit word 'Yuj' that means 'union' or 'joint'. As the name suggests so does the traditional spiritual activity reflects the true life's philosophy.