Cut-off wheels are used to alter the shape of metal. Either to remove it entirely or to remove pieces of it. They're not designed for precision work. They leave a rough surface that will need to be deburred and smoothed before the components can be used in their ultimate application. Cut-off wheels make tiny, accurate cuts at 90-degree angles, whereas grinding wheels employ an abrasive to grind big amounts of material of a workpiece from a shallow angle. As a result, these wheels are frequently thinner than grinding wheels; with more precise cuts.