Well it’s not actually clear to me whether the soft water is to protect the dishwasher, or to make cleaning more effective. Soft water dissolves soap better which makes it more effective in cleaning. It also means I can use powdered detergent (which is cheaper than liquid detergent, but in hard water powder doesn’t perform as well). Soft water has the down side that it’s actually /more prone/ to corrosion than hard water (at least according to youtube plumbers). So I’m tempted to conclude the built-in water softener is just for cleaning effectiveness.