I agree with Michael on this, and feel that it extends to many modern day texts also.
Most (engilish translations of) chinese books that I have read on acupuncture offer little, if any, explaination of why certain points work or why certain points are chosen. A modern example of this can be seen in the book Chinese Acupuncture and Moxibustion** which describes CV 21 as being used for:
" Indications: Pain in the chest, cough, asthma"
and ST 36 for:
"Indications: Gastric pain, vomiting hiccup, abdominal distension, borborygmus, diarrhea, dysentry, constipation, mastitis, enteritis, aching of the knee joint and leg, beriberi, edema, cough, asthma, emaciation due to general deficiency, indigestion, apoplexy, hemiplegia, dizziness, insomnia, mania."
This book offers no rationale and suggests to me that it is taking a more traditional view of acupuncture and describing the points uses in terms of how they move Qi and Blood.
We can theorise till the cows come home about why the Chinese don't tend to offer rationale for their point selections in their literature but that will get us nowhere. I personally welcome the modern herbal type descriptions of acupoint functions as long as it is all thought about thoroughly.
As Michael was describing CV 21 may well be indicated for Food Stagnation, but we don't know if this is the case in all instances or only when an appropriate point combination is used.
Deadman seems to base the point functions on classical formulae, which is logical, but do the points work in isolation the same as they do in a formula? Herbs don't, so why should acupoints?
I personally feel that, as CV 21 is not listed in the classic texts as being able to treat Food Stagnation on it's own then it's inclusion in the combination with ST 36 is most likely due to it's ability to alleviate pain in the chest. The symptoms of Food Stagnation include chest pain or discomfort, so ST 36 treats the majority of the symptoms and CV21 helps by relieving the chest pain.
I think that it is important to consider why the points work from both a traditional Qi and Blood moving view as well as how this relates to the more modern 8 methods type approach... we are, after all, practising holistic medicine, it should theoretically all make sense!
Cheng, X. (1987) Chinese Acupuncture and Moxibustion. Foreign Languages Press, Beijing.