They are not water softeners. They do not remove anything from the water.
They are anti scale - descale devices and are supposed to make water act as if it is softened water. I have seen a couple 'work', they caused a white film like baby powder all over everything that the water was allowed to dry on.
Most do nothing but lighten your wallet.
23 years in water treatment and well pumps, 13 years on the 'net helping others to help themselves.