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Believing that your player character is actually cataloguing and uncovering new information about these species by filling in their Pokedex on their adventure, as though it's actually of some scientific benefit, is like believing in Santa.
It's just something to motivate you, though because the Dex only gets full entries when you catch a Pokemon, it would still serve as an in-universe collector's item if it's full. It proves you've caught a lot of different kinds and been all over.
I'm sure there are kids out there with jailbroken pokedexes wowing all the youngsters in the pokemon schools