Hi Upayan, the short answer is because you are relying on your body to support yourself rather than a chair or other object. Using the body as it was designed allows muscular tension to be distributed more evenly. For example, if you're sitting on your sit bones with a lengthened back and your head is poised and leading you up, then you're encouraging the body to work as it was designed--like you did as a child. However, if the body is not used to sitting without back support, then after a while the back becomes fatigued which leads to slouching or arching the back, which adds more tension to the body--and leads to imbalance. This can also result in back pain. When the body is balanced, it functions more optimally leading to more energy. When it doesn't, it can lead to fatigue and pain. Hope that helps clarify:)