As a tradition, platycodon is used in many illnesses that are located at the chest and upward. For example: cough with alot of phlegm, a sore throat, or even lung abscesses. In many formulae, its role is as a carrier, to bring the other herbs to the chest area.
One modern explanation for this: Platycodon contains many saponins, which put simply are soap-like compounds. These saponins come in contact with a big part of the respiratory tract as well as the digestive tract (of course). And assuming (I don’t know yet) the action of saponins ends at the stomach area, then it is restricted to the chest and upward.
One example of its use at chest level: One particular type of saponin, platycodin (桔梗皂苷), can stimulate the mouth, throat and stomach lining upon contact, and this stimulus is passed on as a reflex, increasing bronchial mucus membrane secretions. This thins phlegm, explaning platycodon’s use as an expectorant.
Platycodon is also a natural aspirin: It is an anti-inflammatory pain-killer that also brings down your temperature. Small wonder its use in A Treatise on Warm Diseases (《温病条辨》). Go look at xing su san (杏苏散) and sang ju yin (桑菊饮) and the host of other cold, cough and flu formulas. It is seldom that platycodon is left out of the equation when treating pulmonary conditions.