What is the role of spark plugs in a car engine?

Look at a spare spark plug. It has two very tiny metal tips separated by a small gap. Spark plugs as the name suggests provide sparks for ignition of the petrol in the car engine chamber. Spark plugs are connected to the battery. When the car ignition key is turned on, the voltage from the battery reaches the spark plugs. One metal tip is at a positive terminal of the battery and the other is connected to the negative terminal of the battery. The electric field developed between the gap is very high and the air around the gap gets ionized. This gives rise to a tiny spark. The spark is sufficient to ignite the petrol-air mixture in the engine chamber. Over time the spark plugs tips wear off and blacken. These then have to be replaced. Thus the role of the spark plugs is to start combustion of the petrol - air mixture.