Loot boxes are in vogue right now, with PC and console titles like Hearthstone, Overwatch and Star Wars Battlefront II offering randomized rewards that can impact gameplay in some cases. The latter game has come under fire by consumers (and governmental bodies) who perceive the implementation of loot boxes as inherently unfair and possible addictive like gambling. While EA has taken steps to fix the system in Star Wars Battlefront II and Need for Speed: Payback, the practice and perception still exist, hence the new Apple rules.
As noted by Polygon, the regulations are similar to those in China, which require game developers to post the probability of getting specific items as well as the actual items received on the titles' official websites. While Apple isn't specific on how such information should be communicated to players, having the rule should at least help let players know a bit more about what they can expect from these loot boxes. Whether that changes their behavior or not is another matter entirely.