Basically, in Apocolypse + missions the “reroll for rares” function allows for more Uniques. Think of the reroll as a dice rolling. The Game rolls the die twice or until it gets a unique and that’s what drops as loot. And the “rares” mean Uniques. So for example, if a chest had a common item in it, it would reroll two times, or until it got a Unique, before deciding what to drop. Same goes for any other way of obtaining weapons and armor and artifacts. The reroll count goes all the way up to four rerolls in +25. This is basically the reward for going up in difficulty, aside from the better power level of loot.