The crankshaft undergoes high loading due to the fact it transforms the linear loading of the pistons into a usable rotational motion. In time it will wear. Rather than replace this relatively expensive forged item the surfaces can be reground.
This regrinding is a specialized process as the grinding wheel needs to travel horizontally while the crank rotates. As in animation.

Regrinding of the crankshaft

The crank grinding machine moves the grinding wheel horizontally the same distance as the crank offset.