Content provided by Michiel Meyer, hosted with permission

If you expanded the neck when loading a lead bullet, then this step is absolutely necessary as it will crimp the case mouth closed again. It can also slightly improve reliability of the round if you crimp the case mouth to the bullet, even when using a CMJ or FMJ type bullet. Be careful not to over crimp the case, as it might actually cause a loose bullet in the case. I prefer to use a taper crimp die for this process. Lots of shooters prefer a factory crimp die.