The motivation to spend extra money for environmental reasons only is often not there, even for so-called environmentalists. Why would a new family whose budget is very tight choose to spend money on a system to reduce their mains water consumption, if the mains water was available for free anyway?
This question however is more relevant if short term economics prevail in our thinking and we have little thought of long term issue of resource sustainability. So many of the water ideas on the Herr Ltd web site are indeed hard to justify for cost reasons alone and also because the planning authority may not insist on them or even recommend them in the first place.
My own motivation for building rainwater harvesting systems is based on my own reading about those societies in the past that were careful to manage their resources sustainably. The writer Jared Diamond claims that these societies that did manage their resources sustainably prevailed while other societies that were wasteful of their finite resources gradually went into decline. Covered in his TED Talk: Why do societies collapse, below.
The example of Easter Island in the middle of the Pacific Ocean is often given as an example of how a society can, over generations, waste or destroy the resource on which the society depends. The cutting down of all the trees on the island eventually resulted in the collapse of the Easter Island society. Our modern world can potentially do likewise if we continue to be wasteful of our water and continue to deplete the remaining world resources of phosphorus.