Yoga is a term which refers to the union of the human with the God or the Self with the Higher Self. This term is many a times used to refer physical exercise or just Yoga postures (Yoga asanas) as many commercial yoga masters are coming up with the idea of making money with this magical word. Yogasana is a part of the bigger picture. According to the eight fold path of the great saint Patanjali, Yogasana is one of the eight components that would aid in achieving the state of union of the Self with the Higher self.
A good health would be a natural result if the Yoga happens. Yes, it has to happen. We can just facilitate the process through some efforts like meditation.