[bigbugbag]

Drinking water does not fall from the sky, in part of the world where there are factories, motor vehicles, power plants, lightning and other cause of air pollution rain water is usually contaminated.

It may be considered safe to use to water your lawn unless it is too acid, but you should not drinking without treating it first. Especially if it has been collected and stored in a reservoir.

Then again food literally grows on trees, you just have to collect it.

Point is most of the time dishwashers do not run on rain water but on drinking water which is a scarce and vital resource so we ought to avoid wasting it.