Why does Lego almost always put blue tanks on fire trucks?

I've noticed that since 2022 a lot of Lego fire trucks have these large blue tanks on the sides or back, supposedly representing water tanks. I know it's a toy, but I've never seen a real fire truck with visible blue water tanks. AFAIK, the tanks were usually hidden inside the body of the truck or the truck is a big red tanker. Why did they choose this as the default? Why do they need to include specifically this in almost every recent fire truck set? Is this based on some real world design...