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 cough with hot phlegm as can be seen on Blackberry Lily Rhizome's page.
Blackberry Lily Rhizomes is a Cold herb that tastes Bitter. It targets the Lung.
Its main actions are: Clears Heat. Detoxifies Fire poison especially for sore throat. Resolves Phlegm and lowers Lung Qi
Because it is a herb specifically indicated to treat cough with hot phlegm as can be seen on Catechu's page.
Its main actions are: Clear Phlegm. Clears Heat and drains Dampness. Applied externally for stopping bleeding.