It’s all about airflow
Roof ventilation is based on the simple fact that warm air rises. In summer, the sun heats air in the attic. In winter, heat from your home warms attic air. In either season, good venting occurs when cool air can enter the attic near the eaves and exit near the peak. Ideally, half of the vent area should be low and half high.
Half of vent area should be high Air flows along underside of roof Ice dam Pooling meltwater Water condenses from moist air Insulation blocks airflowInsulation blocks vents Damp wood and rot Air leak