Some people do yoga for the physical health benefits such as: Improved posture, regulating blood flow in the body and alleviating tense muscles and the pain caused from them. It also helps release toxins from the body.
But there are also some mental and emotional benefits as well. Examples include: Better concentration and stronger equilibrium. Relieving stress (which can also take a physical toll on the body) is a big one. Also it's a way for the body to regulate itself in wakefull times and allow one to sleep better.
Also yoga can be a spiritual experience for many. It can be combined with meditation (or a form itself) to produce some extrodinary experieces. For instance, repressed memories, early childhood memories thought forgotten, and even past life memories.
While many people partake in what I like to call "General Yoga Practices" for whatever reason/s they may choose, there are also techniques which have specific purposes. There are hand positioning techniques and poses used for opening and accessing certain chakras. And breaths for releasing kundalini energies and aiding in awakenings.
Anything you would like me to look into or post on any of these topics, or anything else for that matter, please do not hesitate to ask either on the thread or via pm. I'll do whatever I can to find a method to aid you with whatever the case may be. I personally find yoga to be a highly under-utilized medicinal tool, even though many doctors and therapists advise it to their patients. Try it... can't hurt... much