Actually, the nutrient needs of the plant aren't any higher but the nutrient content of the water is lower!
Both Calcium & Magnesium are typically present in adequate quantities in water, so the nute manufacturers don't put those minerals in their formulas in sufficient quantities. However, both the R/O and distillation process strip those minerals out, along with all other salts, so it needs to be added back in.
I use filtered tap water, but the R/O or Distilled is usually preferred by growers since it takes out the chlorine/chloramines which kill the soil biology; and also, when starting at a zero reading on a TDS meter, you are 100% in control of what the plants are getting. If, for example, you were to start with 150 PPM water, it would likely be fine BUT, you would have no way of knowing how much of those 150 PPM's were calcium vs. magnesium, vs. anything else without some lab analysis.
Rain water is fantastic as a natural water source and doesn't usually require Cal-Mag to be added unless the plant does have an unusually high need.
BTW, Happy New Year, everyone!