"Astral travel" is a by-product of chi kung or meditative practice. It is also a myth. A myth in the sense that there is a far better explanation of the phenomenon than our soul travelling outside our body. My explanations come from my knowledge of psychology (I majored in psychology in University, with continual personal study of relevant subjects after graduation), my study of experiential writings of chi-kung and meditation practitioners (and those who have no practice experience but have astral travel experience, due to heredity, or special make of one's mind/body), my study of Taoist classics, my discussions with those friends of mine who have such experience, and last but not least my own empirical experience.
For certain special people, it comes naturally with vivid dreams or vivid imagery during their waking lives. For most people, such vivid dreams (or vivid imagery during meditation) comes from learned experience of deep meditation. The images (or experience) are always vivid. The person seemingly have logical thinking during such experience, so much so, the person sometimes can logically argue (and therefore convinced) that they are not in the waking stage but is in a different zone (whether it is vivid dream ("I am dreaming') or "out-of-body" experience ("my soul is traveling in different time/space") depends on the belief system of the person). Oftentimes, images can be remembered vividly after waken up. Some experience can be so vivid that details can be richer than what can be found in reality (for example, the image or personality of a friend can be in more details when compared with real life experience). Such experience sometimes can be proved to be that it can help to solve everyday logical problems (not too surprising though, when people without such experience sometimes claim that after a good night sleep, difficult problems have been solved).
It is interesting to note that religiously minded meditators sometimes believe that such inner experience is more important (and sacred) than everyday experience. Not too surprisingly though, since when a devoted meditator meditates daily in secluded places, nothing interesting really happens around him. His vivid dreams draw rich and interesting resources from his past experience, including both conscious and unconscious, the latter including that, for whatever reasons, has been suppressed from his conscious thoughts.
As I said in the beginning, chi-kung and meditation practitioners can have such experience as a by-product. And as by-product goes, it should not be a major training objective of one's training. For me, it serves the purpose of an interesting psychological construct which can help me understanding myself better, in particular any suppressed unconscious thoughts that may surface to my consciousness during my vivid dreams. If a practitioner takes it light-heartedly, he will not become superstitious.