Not all machines can use dishwasher salt and not all areas need it. It’s apparently for built in water softening. In the US for example areas with hard water typically have whole home water softeners.

I live somewhere with soft water so dishwasher salt isn’t needed. I used to live in Ohio where the water is generally fairly hard, and there every house had a water softener. That said if you don’t have a water softener or soft water (or you just get mineral buildup in places with water) check if your dishwasher has a salt compartment