The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
Because it is a herb specifically indicated to treat dermatosis as can be seen on Chinese Gall's page.
Chinese Gall is a Cold herb that tastes Pungent and Sour. It targets the Kidney, the Large intestine and the Lung.
Its main actions are: Astringes Lung Qi and clear Lung Heat. Astringes the Intestines and stop diarrhea. Stop sweating, bleeding and exudations.
Chronic cough Chronic diarrhea Dysentery Chronic hemafecia Rectal prolapse Excessive sweating Spermatorrhea Sores Bloody sputum Abnormal uterine bleeding