Maybe it picks up dirt particles from the air. In my experience, stuff that gets rained on can sometimes acquire dirt that it wouldn't otherwise have. If ky clothes are drenched from the rain, I throw them in the laundry instead of considering them clean.
No doubt, but presumably those non water things are all things that evaporate, so wouldn't get left behind then the rain dries. The rain would have to pick up something up on the way down for it to contain something that precipitates out. That could be dust or soot, I'm not sure what else, but any minerals would have to come in that way
[1] https://en.wikipedia.org/wiki/Rain_dust