If you live in an area that has high mineral content in the water it can be somewhere between highly practical and very necessary.

"Hard" water primarily causes scale buildup, as well as changing the effectiveness of soap and other things that interact chemically with water.

In hard water areas if you like lather from your soap in the shower a water softener is necessary. If you like your white articles to not come out looking gray and dingy you need soft water. If you like long appliance life from your dishwasher, washing machine, and hot water heater, a water softener is necessary. And so on.

If you don't have very hard water it becomes less necessary. Still nice to have but a little tedious to maintain. But in very hard water areas, yeah, you want one.