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 skin infections as can be seen on Knotgrass's page.
Knotgrass is a Cool herb that tastes Bitter and Sweet. It targets the Bladder.
Its main actions are: Expels Damp-Heat and encourages urination. Drains Dampness and stops itching. Kills parasites.