Race car helments have different approvals (ie those for racing) this means if you do any racing, hillclimbing, sprinting etc. that requires certain standards of approval a motorbike lid will not suffice.
Race car helmets tend to have a narrower visor as well.
Some are designed for windscreenless cars or single seaters which may mean they have a fixed or removeable "duckbill" which prevents the helmet from being pulled upwards by the airflow. This can be a serious problem if you are going 100+ on the track.