What's more, yoga improves:
Musle tone.
Flexibility.
Strength and stamina.
Reduces stress and boosts self esteem.
Improves concentration and creativity. Lowers fat.
Improves circulation.
Stimulates the immune system.
Creates sense of well being and clam.
And that's just the suface stuff. In fact most of the benefits mentioned above are secondary to yoga's original purpose.
Developed in India, Yoga is a spiritual practice that has been evolving for the last 5,000 years or so. The orginal yogis were reacting, in part to India's ancient Vedic religion, which emphasized rituals. The yogis wanted a direct spiritual experience. One on one. not symbolic ritual. So they developed yoga.
Yoga means "union" in Sanskrit, the classical language of India. According to the yogis true happiness, liberation, and enlightenment comes from union with the divine consciousness know as Brahman, or with Atman, the transcendent self. The various yoga practices are a methodology for the reaching that goal.
In hatha yoga for exaple postures and breathing exercisies help purify the mind, body and spirits so yogi can attain union. Pranayama breathing exercises help clear the nadis or channels that carry prana the universal life force allowing prana to flow freely. When the channels are clear and that last block at the base if the spine has been opened, Kundalini rises through the sprine, through the central tradition, the release of Kundalini leads to enlightenment and union.