What are probiotics? Why do pets need them? Probiotics are beneficial bacteria that support a healthy immune system. An animal's body needs the right bacteria in the intestinal tract to absorb vital nutrients to fight illness and disease. Antibiotics often wipe out good and bad bacteria. Special formulated probiotic supplements for pets work to restore important micro flora to a balanced state. Not all probiotics are equal. The top manufacturer in the United States is Natren.