Nagura stones help stir up the abrasive in the stone, creating a slurry that aids in cutting during sharpening, as well as prevents the blade from sticking on higher grit stones. During sharpening, you can scrub the nagura stone in a circular pattern to free up the swarf (metal bits) from your stone when it becomes clogged from sharpening. Additionally, it can also be used as a fixer to soften the edges of gouges and imperfections on your stone.