Motorsport helmets designed for saloon car and GT use will usually have a (full face) chin bar fitted. This isn't always required when a car has a roof - in rallying for example.
Most track racing series' require a full-face helmet, but there are subtle differences that define a Saloon/GT helmet from a formula or rally model.
While Formula helmets are designed to be exposed to the elements (they can be used inside a car with the correct homologation), the ability to fasten the visor shut is not required. Saloon and GT car helmets usually feature a sun visor or can be fitted with a short visor for use in bright conditions.