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Mechanically speaking, 'postgame content' is no different from gating off Ecruteak until you have the Plain Badge or the old man blocking the way to Viridian Forest—arbitrary as it is, it's just how Game Freak makes sure the player's scope never extends into something their team shouldn't be equipped to handle (we could discuss whether or not that's a good idea, but that would be a 'should the player be allowed to fight level 50+ wild mons from the get go' question, not a 'is postgame content good' question). Some postgame features, particularly minigames, probably should be available earlier on (or should just have more gates interspersed throughout them), but setting aside certain parts of the game for people dedicated enough to complete the game and come back for more is an elegant way to leave casual and serious fans feeling like they've had the 'full' Pokémon experience even when the former only engaged with a portion of the entire package. Besides just extra challenge, this also allows Game Freak to insert nonessential (but fan-pleasing) segments like the Cynthia cameo, Delta Episode and Rainbow Rocket subplot into the game without interrupting the flow of the main campaign.