What is the biggest benefit of cool down in physical education?

The biggest benefit of a cool-down in physical education is gradually reducing heart rate and blood pressure to prevent dizziness or fainting and promoting recovery.

While other benefits exist (like improved flexibility and reduced muscle soreness), preventing injury and aiding the body's natural return to a resting state is the most significant and impactful benefit of a proper cool-down. A sudden stop to intense activity can be detrimental.