One of the major goals in Pokemon Brilliant Diamond and Shining Pearl, as set out by the long-standing series slogan, is to catch 'em all - acquire every Pokemon in the game. There's no major reward for this unless you're into shiny hunting (though they nerfed the Shiny Charm in BDSP anyway), but the inherent satisfaction of a full Dex is what drives many players to get their hands on every critter in the game.
Related: Pokemon Brilliant Diamond & Shining Pearl: Shiny Hunting Guide
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