Dave,
The theory is, in humid climates, handgun or rifle ammo stored in leather especially, but also in cloth loops often begin to oxidize and you end up with a band of verdegris on the body of the case that makes it difficult or impossible to chamber. Many people just leave it in the loops from one season to the next. Nickled cases don't corrode.
The handgun ammo became popular first with law enforcement as many in that line of work become lax with the maintainence of their revolvers and it sometimes became difficult or impossible to eject fired cases and reload. This cost lives.
It is not an uncommon sight even today to see would be hunters at the range to check their sights before deer season scraping off their cartridges with a pocket knife as they remove them from a belt slide and load them into their rifles. Here, as in many other places, we're likely to encounter rain, snow, sleet, freezing rain, hail and miserable combinations of all of them in the course of a single day during the November deer season. Actions get soaked and so can the rounds in the magazine, worsening the problem. The steel is usually protected by a light coating of oil, but not the ammo. If a man hunts out from his back door, he might not need to unload to get into a vehicle, and just set the loaded rifle behind the door at night. In an unlucky season, the rifle may not be unloaded until more than a month has past. At one time, I used to earn a good bit post season in the weeks before Xmas undoing such things for others - removing rust, spot bluing, cleaning and relubing internal parts.
Now, I load a lot of custom hunting ammo in nickled rifle cases for friends and hunting buddies, maybe a couple hundred rounds of it every year, in .243, .30-30, .308, .30-06. We don't use it for practice or target work except to check sights, for that we buy regular cases in bulk.