The water that does not evaporate during firefighting will remain or leave the fire site. Contaminated run-off water can, depending on soil conditions etc., infiltrate the soil or through surface run-off reach areas that are environmentally sensitive.
This means that run-off water not only damages the materials in the room of origin of the fire but can also damage larger areas of the building or contaminate the surroundings.
Using Cobra leaves less water to cause water damage or contaminated run-off water to harm the environment.