Distilled or Regular Water

So Im curious and looking for opinions. For as long as I can remember myself and others I know growing up used just regular tap water to mix with their coolant. I recently picked up a Aluminum Radiator and it said to run distilled water only. My block still has some regular water and coolant in it. I just want to make sure I dont hurt anything, but Im still curious as to the purpose of using distilled water.