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 genital yeast infection as can be seen on Cnidium Seed's page.
Its main actions are: Applied topically it clears Damp-Heat on the skin and kills parasites. Tonifies Kidney Yang. Warms and dries Wind-Damp-Cold.