Original and genuinely made to store electrical energy and supply power when the engine isn’t running.
It starts the engine by sending a large burst of current to the starter motor to crank the engine over. Once the engine is running, the alternator takes over to power the car and recharge the battery.
It also stabilizes voltage for the electrical system and powers things like lights, radio, ECU, and alarms when the engine is off.
Think of it as the car’s ignition starter and backup power bank.