Crankshaft grinding is done because the crankshaft wears over time. When we grind a crankshaft the journals end up having a greater radius which by reducing stress at the corners.
After the crankshaft is ground to the appropriate size it is then polished. The journals are polished using a piece of emery paper or polishing belts. Polishing the journals prevents any sort of rough finish for the bearings to catch on. The journals become smooth which reduces resistance and improves overall engine power.
Crankshaft grinding is an important part of engine remanufacturing.