The size of the teapot has an influence on the taste of tea. Thus, small and medium sized pots are particularly well suited for teas like sencha or gyakuro, medium sized cans for fukamushicha, ujicha or ureshinocha, Whereas large pots are especially recommended for bancha, houjicha or genmaicha.
In a kettle, you can boil the water directly. That water, boiled with a teakettle, is rich in iron and therefore beneficent for your health. It is particularly effective for use against anemia. For this reason, Japanese put a special emphasis on the use of a teakettle.