This is hilarious.

What exactly is in this magical rain water other that H2O?

And why does it need to be stored in dark containers?

It's not hilarious. Water is serious business - especially when it
comes to tea. And any water is not merely just H2O. There are many
many kinds of water - from many different sources. Would you use muddy
pond water to brew your tea? Or would you prefer swamp water, drawn on
a sunny day? Rain water is actually good for tea - provide it's clean.
But nowadays, with pollution and everthing, maybe it's not the best
choice of water.

Of course, you can always rely on Tapwater Springs - and buy those
bottled waters that are really just filtered tap water. Yuck.