If the system is properly treated with a chemical protectant, it would not be necessary. However, it is some of the cheapest insurance that can be installed in the system.
How many times have we all seen systems that were installed correctly, filled with good water and proper chemical protection; then some part fails, the system gets drained and the owner and/or in repair person just fills it up with potable water and walks away, setting up the potential for all sorts of failures.
Install the dirtmag!