People practice Yoga for many reasons. Some want to have a strong, flexible body and limit themselves to physical, postural yoga to get that. Others want to feel healthier and less stressed, and practice postures, breathing, and perhaps some relaxation practices for that.
But the ultimate goal of Yoga is a spiritual, not a worldly one.
It is coming to know the great mystery, the source of all love, happiness, and energy that is the very core of our being. These aspirants practice the whole of Yoga, all eight limbs.
Through this practice they receive what the partial practitioners are reaching for and so much more.