If someone ask you this question, while drinkable water for human being ‘needed’ 80ppm of impurity. You have to know that it is because of osmosis that we should drink 80ppm impure water, so that water can enter into our body tissues.
while boilers need 0ppm pure water because impurity deposits at the bottom of the boiler, which cause reduce in heat exchange. this is known as fouling.
chemical impurities like O2 and CO2 also should be removed, so that Boiler corrosion can avoid.