Rainwater is technically distilled, as it is produced by a natural distillation process. Heat from the sun causes water to evaporate and rise into the atmosphere, where it cools and condenses, forming tiny water droplets that make clouds. Eventually, these clouds release the water in the form of rain.
On its way down to earth, rainwater will come into contact with a number of airborne pollutants, which means that it won't be as pure as manmade distilled water once it reaches the ground.