[Singletoned]

No filter jug removes all the limescale in hard water areas. You still need a filter on the kettle

[ALittleLight]

Why would a filter on a kettle remove what a filter on a jug doesn't?

[developer92]

A jug filter is much bigger and uses a different mechanism, a kettle filter is just a mesh to anything bigger than a large grain of sand in the kettle.

[sneak]

Because the concentrations in the kettle increase locally due to boiling, and there is precipitate that doesn't happen in the filter jug.