The nickel plated brass is brass cases with nickel plating. Not steel cases with nickel plating.
In the "old days" of carrying your spare/extra ammo in the leather cartridge loop belts or in dump pouches, or even in speed loader pouches, the brass cases would corrode (pretty, or not so pretty, depending on your view) green color of corrosion called verdigras (never looked it up to see why it was called that) and it could build up enough to keep you from being able to insert the cartridge into the cylinder of your (police/personal) revolver. You needed to take the cartridges out and wipe them off/keep them clean on a regular basis. The nickel plating was more resistant to corrosion from contact with leather.
Most of us would find it hard to believe but I've seen this with my own eyes back in the early 80's when I was in LE. I've seen an officer open up his dump pouch and dump 6 rounds of .38 specials into his hand that were absolutely green with corrosion. I've seen an officer try to get his revolver cylinder open and then finally walk over to a brick wall and smack the cylinder on the brick to pop it open, it had been so long since he'd properly cleaned/lubed it that the either the cylinder latch wasn't working properly or the crane had rusted into the frame.
In today's semi autos this probably isn't as important since the spare cartridges are in a magazine in a pouch that is probably nylon or kydex instead of leather, so no cartridge/case contact with leather equals little or no corrosion over time vs. the "old days".
If you're the type who allows his gun/magazines/cartridges to linger to the point of not being usable without doing proper inspection/cleaning/lubrication you probably shouldn't be in LE or allowed to own/carry a fire arm. You're more of a danger to yourself and others armed than unarmed. Just my opinion, but it seems to be a good one.