Most people in Hong Kong don't drink tap water (or at least not without filtering and/or boiling it first). Offices very often don't have their own water supply (hence the slightly bizarre sight of people washing cups in the washroom), so the only option is distilled water, distributed around Hong Kong in large lorries.
Although the water itself is doubtless very pure, the containers in which it is distributed can get very dirty whilst they transported around Hong Kong and left on the street prior to delivery to the office building (because the lorries cannot park for long enough). If you don't clean the top of the container before putting it in the dispenser there is every change that the dust and dirt will get in with the water.
Then there's the fact that most dispensers offer a choice of ice cold and boiling hot water. If you want your water at room temperature you end up mixing the hot and cold together, which is a ridiculous waste of energy.
I have an idea - what if we had pipes to distribute clean and pure water to every office in Hong Kong? No, it would never work...