The Extreme Randomizer is for players who want a —a ROM you can load and know it will always be unpredictable, but also fair within its own chaotic logic.
Static legendary encounters (Groudon, Kyogre, Rayquaza, Regis, Latios/Latias) are also randomized by location and species, meaning the Regi puzzles might unlock a Mewtwo, and Sky Pillar might hold a Bidoof (though likely with Wonder Guard).