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 spots in the vision as can be seen on Vitex Fruit's page.
Its main actions are: Relieves the Exterior, scatters Wind and clears Heat. Clears Wind-Heat associated with the Liver. Clears Wind-Damp painful obstruction.