Weedhound's right, there are unwanted contaminates in melted snow just the same as in rain water. The closer you are to major industrial areas, the worse the pollutants become.

And CalMag supplements the Calcium, Magnesium, Iron, and trace elements that aren't present in distilled and RO water. Normally, when using tap water, these elements are present and in enough quantities to prevent deficiencies... but in distilled/RO water, they've been removed.