Tap water contains chlorine & sometimes fluorides wherein rain water contains nothing.
The difference is yes tap water has a few chemicals to purify it but rainwater isn't "clean"or natural for that matter. Think about this rain water comes from clouds,clouds come from water thats everywhere not "just the sky" look up the water cycle and you should notice that water and gases make up clouds. Clouds also go over highways and major pollution areas therefor making that "clean" water polluted.
Tap water is rain water that has a bit of processing done to it after it is collected in a lake, stream, or aquifer. Take a look about half-way down this page.
Tap water contains chlorine & sometimes fluorides but rain contains nothing
Rain water is natural duh