The materials, briefly
Stainless steel is the default: inert while intact, durable, and it hides scale so you find out late. Glass is inert and shows everything, including exactly how much scale you have, which is a real advantage. Enamelled steel is a glass lining on a hob kettle and is fine until it chips. Unlined cast iron is a different object with a different practice and is covered on its own record. Plastic kettles and plastic-bodied elements are common; TeaHQ assesses materials only for how they behave in use and takes no position beyond that. The choice between these changes the water far less than what follows.
Scale is a chemistry event, not just a deposit
When hard water is boiled, dissolved calcium bicarbonate breaks down and calcium carbonate comes out of solution — that is what scale is. Two things follow. The water leaving a kettle in a hard-water area is less hard and less alkaline than the water that went in, because some of the hardness is now on the element, which means boiling is itself a partial softening step. And a heavily scaled element heats unevenly and takes longer, which matters for a kettle you use twenty times a day. Descale on a schedule rather than when it looks bad.
Reboiling, and the dissolved gas argument
Water holds dissolved gases and loses them on boiling. Water that has been boiled, cooled and boiled again holds less, and many drinkers describe repeatedly boiled water as tasting flat. TeaHQ can state the gas loss as a physical fact. How much of the reported flavour difference follows from it, as against from dissolved minerals building up in a kettle that has been boiled down repeatedly, is not something it can source. The practical advice survives either way and costs nothing: draw fresh water and boil what you need.
Held-hot water is a separate case
A boiler or thermo pot holding water at temperature all day is not doing the same thing as a kettle. The water is above ambient for hours, it has lost its dissolved gas, and it slowly concentrates as some evaporates. This is a genuine convenience and it is the arrangement most likely to be behind a complaint that tea at work never tastes like tea at home. Emptying and refilling rather than topping up addresses most of it.
What the kettle cannot fix
Nothing in a kettle removes chlorine compounds, changes alkalinity meaningfully, or improves water that is unpleasant to begin with. Those are filtration and source questions and are set out in the water guides. If tea tastes wrong in every vessel you own, the water is the first suspect and the kettle is nearly never the answer.