So, what is the requirement of water at home?
I come from pathology background -- naturally, I would look at water from a laboratory perspective. What is required for laboratory reagent water? In a laboratory, we are looking at several aspects:
(1) Ionic impurities (e.g. >10 megaohm/cm3)
(2) Organic impurities (e.g. <500 ppb)
(3) Microbiological impurities (e.g. <10 cfu/ml), and
(4) Particulate content (e.g. passing through a 0.22 um absolute screen filter)
The first one is almost an absolute requirement, that laboratory reagent water contain as little undesired ionic contamination as possible, as it will interfere with trace analysis (or if the contamination is severe, non-trace analysis -- that would be gross). The impedence of water is used as a way of measuring ionic contamination. Water has a theoretical maximum impedence of 18.2 megaohms -- and when exposed to air, the highest impedence would drop to around 1 megaohm. Water is purified to remove ionic contaminants mostly by the use of ion-exchange resins. To improve the efficiency, often multiple resins are used in cascade.
The second one refers to the amount of organic compounds dissolved in the water. Organic contaminants can be removed by oxidation by UV light, as well as absorption and adsorption by activated charcoal.
Microbiological impurities are often removed by depth filter, and chlorination of the tap water.
Particulate content are usually removed in multiple stages: firstly, depth filter consisting of fibers interwoven to trap particulates in a random manner is used; depth filter is a high-capacity, but non-absolute filter. In contrast to depth filter, a screen filter (absolute filter) is used at the end of the water purification process to obtain an absolute cutoff. However, screen filters are low capacity with regard to contaminants, and thus a depth filter in front of it would greatly increase its lifetime.
So, for human consumption, what is the role of water treatment?
Briefly, water from the water service department in Hong Kong is considered potable - i.e. harmful constituents are at a very low level when the water leaves WSD. What is the caveat? It's the route between WSD and your home. What could go wrong?
Plastic pipe can leak organics. Old pipes (if any) could leak heavy metal ions. Bacteria can grow in the water reservoir on the top of the building. Iron oxide fragments can go into the water, etc. Another problem is the presence of chlorine in the water, which cause a change of taste.
To be honest though, none of this is significant to the extent that it is harmful to drink from the tap. And to make it clear, boiling probably does not help (other than perhaps evaporating some volatile organics). And thus, water treatment system at home is probably helpful - Take a chance, make a cup of tea with tap water, and a cup with distilled water from a reputable manufacturer - it WILL taste different. Why? - The organics, the chlorine content, hardness -- all make differences to the taste.
Thus, in my opinion, water treatment at home is mostly aesthetic (taste-enhancement) and psychological (feeling a little bit safer with regard to heavy metal which is essentially non-existent in HK).
For my home, I consider a depth filter, followed by ion exchange resins (to remove cationic [heavy metal] impurities), and activated charcoal filter to be adequate for health. In fact, for well-maintained housing establishments, this shouldn't even be necessary - except charcoal filter perhaps. Charcoal filter enhances the taste of water a lot.
There is probably little need to use elaborate deionization, osmosis or so on for home water.