Aerodynamic helmets are designed to allow air to move more freely around and over the helmet with the goal of reducing drag. They do this by having smoother surfaces and more sculpted designs than their all-around counterparts. An aero helmet will almost always have less drag than a helmet that isn’t specifically designed to be super aerodynamic, and most have undergone extensive wind tunnel testing to ensure they work as intended. In general, an aero helmet will provide a marginal gain in efficiency, which, when added up over time can result in a more substantial savings of time and energy. That said, aero helmets tend to have a slight reduction in ventilation and a slight increase in weight, comparatively, so that is something to be considered. Still, if saving a few watts and seconds is important to you, an aero helmet can help you do that, and it costs a lot less than an aero bike or aero wheels.