Both personal safety and many sanctioning organizations require the use of switch for easy and complete cutoff of power from your battery. Easy to install and even provides a fused ancillary connection.The most common use of this "great idea" device is as an inexpensive security system. When installed and used properly it is difficult for even a pro to steal your car! The switch is installed in the main battery line thus allowing you to totally disconnect the battery from the system and take the key with you! Almost as good as taking the battery with you when you leave your car.