Every dishwasher has a built-in water softener (ion exchanger). Its task is to "pick up" limescale from the water before it gets on your dishes. For that softener to work, it needs salt. If you forget to add salt:
White spots: Scale deposits on glasses and rims, making them dull and mottled.
Machine breakdowns: The heater and sprinklers get scaled, which increases power consumption and leads to breakdowns.
Worse washing: Detergent does not work effectively in hard water.


