Yes you leave them in the kettle, and clean periodically with vinegar or something.
The theory being that scale will settle on every surface. By providing something with a massive amount of surface area, there should be less available to precipitate out on the kettle surface. The total amount of scale does not change, it just gets spread across more area.
Personally, not worth trading for quick boil with dilute vinegar, which you'll still have to do, only less frequently.