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 chronic hemafecia as can be seen on Chinese Gall's page.
Its main actions are: Astringes Lung Qi and clear Lung Heat. Astringes the Intestines and stop diarrhea. Stop sweating, bleeding and exudations.