Purpose was to prevent corroding when in contact with leather wasn't it? Or is that pistol brass?
Yes, the original purpose was to prevent corrosion. Leather tends to attract moisture, pistol round carried in a leather holster would corrode, and become unsightly in a duty belt, not to mention that once corrosion starts on brass it is pretty much ruined for a firearm. Rifle brass that is nickle plated is more or less designed for harsh enviroments, such as Alaska, Africa, or other places where the ammunition may sit for awhile, or be carried for a couple of weeks in harsh conditions. Take piece of brass, let it get water on it, a couple of drops will do, then let it sit for a few days, corrosion will show up quickly.
Also nickle tends to be slicker than brass, so that feeding and extraction in these environs is much easier.
On the whole, both work well, but I have noticed as have others that nickle is extremely hard to work with at the bench, it plays hell on trimmer cuttters, and dies if the plating is nicked. Although I have not noticed much case stretch with nickle.
I have both nickle and standard brass for my hunting rifles, and have gotten to where I only use the nickle when the weather is going to be very nasty.
Just my 2 cents
DR