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My gut feeling would be to stay as far away from this concept as possible. Rainwater typically contains a great deal of chemicals and pollutants that I wouldn't want in my brews. So let's suppose someone has a different idea and wants to use rainwater. The filtration needed to bring Rainwater to brewing quality would be quite substantial. So I guess I would have to ask: Why would you consider such an indevour?
Rainwater is relatively benign. How you collect it and how it gets channeled to your collection bin/container defines how safe it is for you. If you avoid collection from asphalt roofs, rusty metal trays and gutters, and leaf and debris covered areas you will have the best chance of coming up with clean water. You will want to let it sit covered for a bit to allow for particulates which get caught up to settle out, and make sure that your container is clean and sanitized to start with to make sure there is no growth of coliform or similar harmful bacteria....
Overall, it sounds like it would be much more work and effort that any potential benefits you may receive from use.
About a million years ago, I made a helles lager from flood water as a Red Cross fundraiser for a radio station. The trouble is that the flood water is so fouled that it just didn't seem like a good idea. We were able to collect 40 gallons of clear sky rain water, which tested pretty clean. We collected the rest of the batch water from an aquifer that was filled from the flooded area. We just carbon filtered it and then used it in the beer. It worked great and tested clean. Mother nature does a great job of purifying water, if left to her own devices.
I use rain. It comes of a color steel roof and gutters into a near new plastic tank. It tastes terrific but I carbon filter just in case. Dissolved minerals are only 17ppm so close enough to RO water if you ask me so its a matter of adding a few salts for brewing . Its going to depend where you live I think, how many trucks and industry and just how much rain you get per year to flush through your tank and roof. Buy a cheap TDS meter for 10 bucks and test it. Good luck