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 cholangitis as can be seen on Milk Thistle Seed's page.
Its main actions are: Clears Heat and dispels toxicity. Regulates the flow of Qi in the Liver and tonify Gallbladder.