Select to expand quote
nebbian said...
Fresh water promotes mildew, which rots your sail.
Salt water makes salt crystals, that scratch the film.
Thanks, Nebbian. This is helpful; makes it crystal-clear.

Now I understand the Ezzy answers, which are simplistic. They are Maui-centric. People do indeed sail on fresh water; Lake Garda, the Gorge, Chiemsee, The Great Lakes come to mind.
90% of my sailing is on very fresh water. If I were to blithely follow the Ezzy advice, I would be doing my sails no favours 90% of the time.