What is yoga? Why do people do it? and was exactly is "it" that they are doing?
Yoga has many faces. People are practicing it all over the world. For some it is a deeply spiritual practice, for others a physical workout. The common denominator is that the practice of Yoga is an individual pursuit.
Yoga relies on experience, your personal experience. The more time you devote to your practice the more you experience the changes in body, mind, and soul. Even if you are not looking for those changes they happen. That is the wonder of a yoga practice.
If you approach yoga from a physical perspective; looking for improved flexibility and strength, you will absolutely experience these changes in your body. What you may not be looking for, but will experience, is a positive change in your energy level and your mood. These transformations occur hand in hand with the transformations your body will experience.
The same is true if you come to Yoga with the goal of deepening your spiritual practice. Following the tenets of Yoga you will definitely experience a deeper connection with your choice of Divinity and definitely will experience more joy and peace in your life. On this path you will also strengthen your body and realize greater range of motion and better physical health.
Without an exception all Yoga masters tell their students, "practice, practice, practice." The Sutras of Patanjali expound on this theme also. Why is this a central part of a yoga practice? This is so because Yoga is an experience, your experience. Another way to put it is; yoga is your story and you are writing it, you are in charge.
As the author of your story you may take guidance from your yoga teacher, from the writings of other Yoga masters, but ultimately you are responsible for yourself.