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 venomous bite as can be seen on Realgar's page.
Realgar is a Warm herb that tastes Bitter and Pungent. It targets the Stomach, the Heart and the Liver.
Its main actions are: Clears toxicity and kill parasites. Relieves itching. Heals snakebites and ulcerations. Dries Dampness. Treats malarial conditions.
Abcesses Sores Eczema Herpes zoster Nasal polyps Poisonous snakebite Rashes Ringworm Scabies Skin itching