The science of the Yoga is a lot more than the physical poses. Yoga is a life philosophy that can enrich our lives enormously.
The traditional Ashtanga (Ashtanga means 8) Yoga - as described in the Yoga Sutras of Patanjali - is a discipline that consists of 8 different elements. If you lead your life according to these principles you will lead a happy an healthy life. The Yoga path is a path that leads us to true happiness.
The Yamas and Niyamas are not just about good or bad. What is most important is being true and honest with your true self. If you live according to these principles, you will live a better life which allows you to give direction to your life for a higher awareness of the existence so it becomes easier to evolve from Atam to Paratman.
For a lot of people the Yoga path starts with the third element, the asanas. This is logical as it is the most tangible element of the Yoga science. Even when you are only practicing the asanas for the moment it can be interesting to already become aware of the other aspects of the Yoga.
When you practice asanas for a while there will be a point where you will start practicing the other aspects of the Yoga automatically so you can take your Yoga to the next level beyond the physical poses.