Metal roofs need to breathe. There is sufficient ventilation to allow hot air to escape, making the airflow a necessary condition for a healthy roof and attic.
The effective formula used to determine the required ventilation is to work at a ratio of 1:300. If you have a 300 square foot insulated roof, you must have at least one square foot of ventilation.
This may vary by region, so check your local building codes for guidance.