Yup, water might start out pretty clean at the 'source' but by the time it gets through pumps, reservoirs, mains, and pipes (some times decades old) there's a lotta crap that can end up in your fish tank.
In a sense, a large part of the sanitising additives in the water are to compensate for the crappy pipes they have to travel through!
Even when I was collecting rain water for the tank it is hard to keep it 'pure' cause there is so much other crap blowing around when the rain is coming down. RO/DI made a difference for my tank.