Not a crimp, it's a cannelure, usually seen on bullets but sometimes on premium cases to help prevent bullet setback, as others have mentioned.
Here are some that I have (Aguila, Cor-Bon, WCC, and the last three Federal)
Notice the three Federals all are at different depths. I presume for different bullet weights (230, 200 and 180)
I have a box of Federal Premium 230 grain HST with the cannelure:
