Marketing campaigns for commodity bottled waters try to make us think water must be "pure" to be good. Being clean and healthy isn't enough — water has to be pure, and the purer, the better.
These marketers tell us that nothing but hydrogen and oxygen should be in our water. Ironically, this misconception means people often drink distilled water and in addition mineral supplements, which contain the same minerals that have been removed from their water in the first place.
Even distillation, which removes more contaminants than any other purification method, does not produce thoroughly "pure" water. The rising steam is supposed to leave all impurities behind in the distilling process, but gases, chemicals, and organic compounds can be taken along with the steam. Active carbon filters are used to eliminate those remaining contaminants from distilled water, but some impurity remains with this process, too.
In reverse osmosis (RO), water molecules are forced through a rubber membrane, leaving impurities behind. But gases, some chemicals, including chloramine and arsenic, and some bacteria can beat this technique too. Water treated by either distillation or RO will become acidic upon contact with air — airborne carbon dioxide reacts with the water, taking the place of the removed minerals or contaminants.
So, there is no such thing as pure water? There is a need to clean the wafers in computer chip manufacturing, and water of incredible purity is used not to contaminate the production process. By removing minerals and organic compounds, this ultra-pure water becomes acidic and "aggressive," meaning it will seek to replace the minerals removed. It would be harmful to digest and dissolve a metal tool left behind in the water entirely.
Clean, healthy water doesn't need to be pure. The waters with the most epicurean interest contain minerals and trace elements.