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 mouth bleeding as can be seen on Puffball's page.
Puffball is a Neutral herb that tastes Pungent. It targets the Lung.
Its main actions are: Clears Lung Fire and benefits the throat. Stops bleeding when applied externally.