Yoga can be interpreted in many ways. Many think of yoga as a form of exercise. It is also used to describe a certain spiritual path. It could even be seen to stand for God. The way that I look at Yoga is that it is a path to or experience of unity. At first glance this idea of unity is easy to gloss over as it is used so commonly that the meaning is lost. But I mean unity in terms of the end of one's perceived experience as a distinct, separate individual.
Yoga improves strength, stamina, flexibility, circulation, body awareness, sensitivity, balance, coordination, breath, attitude, endocrine function, digestive function, etc... You get the idea. While some of these aspects of the body and mind can be improved with other forms of exercise, yoga starts with the premis that body, mind and spirit are one. All yogic techniques from posture to breath work to diet are done with the object of experiencing this oneness. It is not necessary that you practice yoga for these reasons, but recognition of this unity will deepen if you continue to practice honestly. In other words, this union or unity or bliss that Yoga refers to is not a mythical, fantastic dream that the naive have imagined. It is a reality that can be experienced by anyone at any level of ability.