Why would you use an insoluble salt to soften water?

The reasons to use insoluble salts to soften water would be to make the water more potable, and to enable better sudsing when washing clothes, dishes, or for taking showers. Hard water tends to decay pipes at a faster rate, leave deposits on sinks, tubs, and toilets. Hard water has a high percentage of calcium and other minerals that have an unpleasant taste and will often cause the joints between pipes to start leaking much much sooner than they would otherwise.