Where it genuinely does damage
The first casualty is the electric water cylinder. Scale settles on the element, forming a crust that insulates, and the appliance has to run longer to heat the same water. You notice it on the bill before you notice it in the hot water, and it ends with a burnt-out element.
Second are the appliances that heat water: washing machine and dishwasher. Third is thermostatic mixer taps, whose cartridge is sensitive and expensive to replace.
Where it is only a nuisance
White marks on the shower screen, a shower head losing jets and a stained sink are cosmetic annoyances. They clean off with vinegar or a mild acidic product, and they shorten the life of nothing.
Worth separating the two, because much of what is sold against limescale is sold by pointing at these visible nuisances, which are precisely the cheapest ones to deal with.
Water softeners: when yes
They make sense where there is a cylinder or a boiler worth protecting, where the property is lived in year round, and where elements have already been replaced more than once. In those conditions they pay for themselves.
They make much less sense in a holiday apartment occupied six weeks a year: the unit needs maintenance and salt, and a treatment device abandoned for eight months is worse than none at all.
The devices that promise results without salt
The market offers magnetic and electronic devices promising to deal with limescale without salt and without maintenance. Results are disputed and uneven, and the independent evidence is considerably weaker than for a resin softener.
We are not saying they never work. We are saying that before spending, it is worth knowing exactly what you expect and accepting that the effect on actual scaling is hard to measure in a house.
The cheap things that do work
Draining and servicing the cylinder every few years, replacing the sacrificial anode when due, extends its life far more than any accessory. It is dull maintenance and it is what genuinely changes the outcome.
And in the washing machine and dishwasher, use the right dose of salt and rinse aid and run an empty cycle with a descaling product a couple of times a year. It costs little and heads off the expensive breakdown.