For handgun ammo it does something that is invaluable--it not only will put whatever crimp you want/need on the round, it post-sizes it so it will chamber, guaranteed. Why do you have to worry about this?
Case length variations can cause problems if you try to seat the bullet and crimp with the same die. I had all kinds of problems on a Dillon SDB with this problem and was always having to run everything through a case gauge. Once I switched to Lee equipment and incorporated the Factory Crimp Die in my loading, slight case bulges due to case length differences were post-sized completely. Now use the case gauge for a paper weight.
