Calcium and pool heaters

Happy spring everyone. After a super long winter it's nice to have the pool open.

One item that I have not found a good explanation of is the need for calcium for pool heaters. I have a Hayward NG heater and the manual instructions are to have the Calcium from 200-400 ppm, and it indicates:

Low - corrosive to heat exchanger

High - scaling of heat exchanger

I understand the scaling with high calcium, but not how low calcium is corrosive.

I read one site where the author theorized:

"However calcium's purpose (in my opinion) is not to prevent the water from being corrosive (as that is what the pH is for), but rather to add a temporary level of protection to the surfaces that come into contact with corrosive water, particularly heaters, metal fittings and concrete pool surfaces. The way calcium does this is by leaving a thin calcium film on these surfaces, when the water balance of pH, alkalinity and calcium is ideal. If your pH does drop below 7 (ideal is 7.4-7.6) the corrosive water has to first etch the calcium film off before it can corrode the metal or concrete surfaces adding a degree of temporary protection to these vulnerable surfaces."

I am not posting the page link because while a good amount of the info mirrors poolsolutions, other is not very good at all.

I keep the calcium as per the manual, but would appreciate any thoughts on calcium use with a heater and why a minimum level is needed.