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 hydrothorax as can be seen on Genkwa Flower's page.
Its main actions are: Relieves chronic congestion of Fluids in the chest and stops cough. Drains congested Fluids through the urine and the stool. Expels parasites.