brooklyn1 wrote:

And I don't know why anyone would need more than 100 gallon tank to
collect rain water for watering some plants around ones abode, and
that's more of a head trip than a real money saver.

Some people have gardens rather than a few plants.

Anyone who lives
where they can keep say a 1000 gallon tank filled with rain water
doesn't really need to be collecting rain water if it rains that
much.

Not true, you haven't thought this through. There are places where annual
rainfall is quite high but very seasonal or very erratic. You need to save
when it rains to water when it doesn't.

This entire concept of collecting rain water in huge tanks
where it hardly rains is really pretty silly... the point of
diminishing returns is reached at about 100 gallons, probably more
like a 55 gallon drum... begins to cost more to transfer and haul
than to turn on the hose bib.

What if there is no hose bib connected to mains supply? What if due to
drought watering gardens from the mains supply is forbidden?

Consider the cost and time of replacing a garden that has turned to dust
compared the cost of a watering system. Of course if you live in an area
where it hardly rains you would have to consider if you can grow a "normal"
garden at all before installing water tanks that will never fill.

When there's a need for large
quantities of water for commercial purposes in arid climes they drill
a well.

What if you are not in an area that has subterranean water?

No one is going to maintain a lawn in say Las Vegas with
collected rain water no matter a 5,000 gallon tank, a lawn will drink
up water in the desert faster than it rains.

Watering lawns will indeed require huge investment in a desert, I for one
would not attempt to grow a lawn in a desert.

David