What is a immobilizer used for in car?

An immobilizer is a security device in a car that prevents the engine from starting unless it detects a valid key. It does this by using a transponder chip in the key to communicate with the car's computer system. If the correct code isn't received, the engine is disabled, even if the ignition system itself is bypassed. This makes it much more difficult for thieves to steal the vehicle, even if they've hot-wired or otherwise gained access to the ignition system.