According to the Taoist text, the Daozan, "the qi (energy) creates the blood creates the jing (essence/vitality) creates the marrow."
It is the marrow that is associated with the yuan qi (source/primordial energy) that the Taoist adept associates himself with, and works with (qi gong). Bone Marrow Washing Qigong is considered the best qigong method for the treatment of immune system disorders. The purpose of Bone Marrow Washing is to move the qi through the bones, since the original qi (yuan qi) is found in the marrow; best for purification (detoxification) and longevity. -paraphrased from the teachings of Zhourenli
Perhaps one reason why the bone marrow is focused upon in practice is because it occupies the center position of the bone, the core physiological tissue. Occupying, holding, or positioning oneself in the center of the universe, or becoming the anthropocosm (cosmic human), or true human (zhenren), or the axis of the world (axis mundi), is the first natural precept of the Taoist adept.
For more info about qigong goto:
To purchase complete Bone Marrow Washing Qigong DVD, please follow payment instructions below. View video by clicking on chapter links on left.