Your head can get pretty hot inside a plastic lid stuffed with padding. Ventilation allows cooler air to enter the front of the helmet and forces stagnant air out a rearward vent. As our editor Jason learned in a recent review of the Bell Eliminator, tabs on both sides of the visor are helpful, as most people lift their shield while holding in the clutch. The level of acceptable ventilation is also region-dependent, as our friends in Arizona would undoubtedly benefit from a helmet with higher airflow. However, do keep in mind that more ventilation leads to more wind noise.