I think it's common knowledge that water affects the taste of tea. 

But did you know that the kettle that boils water also affects the taste of the tea? 

The iron kettle can adsorb minerals dissolved in the water, which can change the hardness of the water.
In soft water, tea components such as umami and astringent tend to leach out of the tea leaves.
It means that you can enjoy the taste of tea as it is with soft water rather than hard water. 

Nambu tekki, a traditional Japanese iron kettle, absorbs minerals in the water and has the effect of lowering the hardness of the water.
There are many iron kettles that are made to look like them, but in many of the similar products, the minerals that have been adsorbed once will leach out again the next time you use them.
However, high-quality Nambu Tekki is made so that once the minerals are absorbed, they do not leach out again, so you can always boil water that is suitable for soft tea.

Reference

Hiroshi HIRATSUKA, Kenji TAKENO, Ken SASAKI; Water Quality Changes with Heating of Tap Water Using Nanbu Iron Kettle and Foreign-made Iron Kettle

平塚広, 竹野 健次, 佐々木 健; 南部鉄瓶および外国製鉄瓶による水道水加熱と水質変化. 日本食品工学会誌, 5 (2), 105-111 (2004)