Failsafes are very important when it comes to nitro vehicles. The device stops the vehicle when it comes across radio interference or if the batteries get low. Without it the vehicle would continue at the last throttle position it received until it hit something or the engine died. The failsafe is connected in line between the receiver and the throttle servo. When a failsafe detects a loss of radio signal or low battery voltage, it will move the throttle servo to a pre-determined position, set by the modeler, usually full or partial braking, to stop the vehicle and prevent damage.
Most electric vehicles do not need a failsafe because modern electronic speed controls include radio priority circuitry, which serves the same purpose, except the speed control just returns to zero throttle, rather than a user-set position.