Normally, sharpening stones should be flattened every time you use them. If you use a harder stone you can get away with truing them less often, but they still build up steel which can clog the grit of the stone and stop it from working properly. Nagura stones are a smoother stone that work like a pencil eraser, removing the built up steel and smoothing the stone's surface. They're also great to smooth your stone after using a rough atoma diamond plate to flatten your stones, but a nagura should never be used in place of a truing stone.