Plants love rainwater...
You should really know what could be in it though. If you live close to crops that get dusted you could have pesticides all in the water.
If you live in the city you could have many things in it from bird poop to rat poison.
Always let it rain for some time before you start collecting. The water should be clear with no turbidity. Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates.
The more total suspended solids in the water, the murkier it seems and the higher the turbidity.
I treat my rainwater with Consan 20 to be safe. I think Im safe anyway