Hurrah, we come to the end of the module! We started by asking why we use fire extinguishers when we can just use water. As you saw, if the fire is an oil fire, water is useless because oil floats on water.
This is why fire extinguishers are used. They are lighter than oil and can stop Oxygen from reaching the fire.
Therefore, even though a fire extinguisher is more expensive than just water, we use fire extinguishers everywhere.