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Berries were introduced in Gen II to provide an in-battle use for Pokemons' newfound ability to carry items. Other items were created to boost damage or other things in battle, either generally (Charcoal) or for a specific Pokemon (Thick Club, Lucky Punch)
They also found a way to expand the Pokedex using items (Metal Coat, King's Rock)
Gen III expanded upon the berry premise to make a wider variety of berries with both general and niche use.
On the topic of Pokemon diet, all Pokemon can consume and digest berries, milk, carbonated beverages, and some herbal supplements. While plenty of Pokemon do eat meat, there is no evidence of them being true carnivores. While herbivores may exist, many Pokemon are omnivorous and likely sustain themselves on mixed diets.
