Bike helmets are a safety precaution designed to protect and reduce impact to your head if you are involved in an accident. They are manufactured to be comfortable enough to be worn at all times and allow you to continue cycling in a normal manor, with no restriction to vision or movement.
Wearing a bike helmet could save your life as well as protecting you from serious injuries. Many years ago bike helmets were cumbersome, heavy and not exactly visually appealing. Bike helmets are now great looking, lightweight and offer better levels of protection than ever before.
Choosing your bike helmet should be based around the intended use. The vents on MTB helmets allow the hot air to escape vertically, like a chimney, from your head. The vents on a road helmet are designed more aerodynamically to allow the heat to be pushed backwards and out of the rear of the helmet.