Chinese herbology is a system of herbal treatment; it is not applied in the way that most herbs and medicines are used according to Western medicine. Under western schools of thought, all medicines, including herbal medicines, are used because they are known to produce certain effects. St. John's Wort and Prozac are used for depression because they are both known to counter depression in the human body.
Chinese Herbal Medicine was developed as an integral part of Chinese Medicine. It is used to re-harmonize imbalances in the body. Therefore, a cough would not be treated by choosing an anti-tussive alone. First, the internal imbalance that is causing the cough would need to be determined, and then the appropriate group of herbs would be combined to treat the problem.
Chinese herbal formulas tend to be much gentler than western medicines, and work to relieve symptoms, as well as, to help restore balance to the body, so that herbs will not be needed further. That is why we often modify the herbs as treatment continues, since the body starts to shift towards being healthier, and therefore the herbs are modified to meet the changing needs of the body.