You've probably heard the phrase "Wear your helmet" sometime in your life. While in reality it's usually for riding your bike or motorcycle, in games, it's usually because you want to protect your precious brain from bullets, plasma, swords, axes, lances, ninja stars, etc. Of course there are many different types of helmets. There's everything from the warrior helmets that legion soldiers wear in The Elder Scrolls IV: Oblivion to the Halo helmet in... Halo. They come in all shapes and sizes so long as they fit on your head. They are also in a lot of racing games when the driver is visible.
Helmets also serve another purpose for things like representing a futuristic HUD. This is prevalent in many games such as the Metroid Prime series. Your perspective is behind a visor and supposedly makes you feel like you are in fact wearing a futuristic space helmet.