Can do a great job at making a player feel like they've improved greatly, the challenge of the harder encounter being followed by the subtly easier one can feel more like they've improved, and gives time to rest for the next one.

Variable difficulty curves in progression design often go undercooked, you'd expect difficulty curves to be fairly straightforward going up through the the course of a game, but rest periods after very difficult encounters or even rewarding players with an easier challenge after a harder one-

I'd be interested in hearing it!