In the beginning, they did it to prevent the green verdigris that grows on brass when carried in cops leather shell loops and drop boxes.
Later on, it was used on .38 Special wad-cutter match ammo to aid extraction of the low powered rounds in target auto-loaders.
(Nickle is slicker then brass)
Then it became a feature of "premium" SD ammo.
Now, I think it is just a selling point.
"Mines shinier then yours!
"Well, mines got black plating on the bullets!"
"But mines still shinier then yours!"
"Now it ain't! Mines got nickle plating now, AND black bullets!"
"Well mine comes in a 20 round box, AND, costs as much as a 50 round box of yours, so it's way better!"
"NO Tain't!"
"Is too!"
"Is not!"
"Is too!"
"Nanny nanny naw naw!"
rcmodel