In fact I tried San Francisco water, and it was completely fine. And there's also the obvious question about the carbon footprint of bringing coconut shells to US for manufacturing.

On the other hand, in several places in Poland a water filter is, actually, fairly useful.

I use a carbon block filter (Clear2o) for SF tap water, and appreciate the lack of chlorine taste. (Or more properly, chloramine: http://www.sfwater.org/index.aspx?page=357 )

In (some parts of?) London, water is particularly hard. A filter makes it not taste of dust and prevents stupid dust spots on container. Some other places have chlorine-tasting water.

For anything else? A filter is pretty much useless.

A significant amount of bottled water is, in fact, tapwater. There are a lot of companies that are bottling your own tapwater and selling it to you for $4/L.