"Yoga is a practical method for making one's life purposeful, useful, and noble." --B.K.S. Iyengar
There are as many benefits of yoga as there are definitions of the word. Yoga is a practice that joins the powers of the body and the powers of the mind through asana (poses), meditation, prana (breath), and savasana (active rest). The practice of yoga can be started at any point in one's life journey--young or old, healthy or sick, fit or injured. Yoga brings focus to the mind and body, builds strength, flexibility, balance, and equanimity. It can be used to heal injuries, calm the mind, cleanse the body of toxins and invigorate the cells, pacify the emotions, and much more.
Yoga is a science of the body and the mind that has been practiced for over 5000 years. It has been passed down from teacher (guru) to student (sadhaka), verbally and through written texts. This lineage can be seen through the different styles of yoga, tracing the form and teachings of the teachers before. Every teacher/style has something to offer students. Finding the practice that is right for each student at this moment in time is part of the yogic journey.
It is consistent practice that makes yoga so effective, the repeated focus of the mind on the body in a concious way, learning to know oneself. Yoga is a process--there is no magic or promise of instant gratification. However, through practice and intention, yoga can bring about health and change in all aspects of a practitioner's life. May you be present and fruitful in your yogic journey.