It is because oil is less dense than water. This means that oil floats on top of water. When a thin film of oil is spread over water, the oil molecules spread out and form a barrier between the water and the air. This barrier prevents the water from evaporating, which is why a thin film of oil can help to keep water fresh for longer.

On the other hand, a thick layer of oil can be harmful to water quality, as it can prevent oxygen from getting into the water. This can lead to the death of aquatic plants and animals.