Games have a way of making players do certain things by compulsion or force. Perhaps that could be considered true for Pokémon games as well in certain respects. One way that could happen is by forcing players to use up certain game items in order to make progress in the games at certain points; this is what I would define as "item traps". Now, I'd like to make the argument that the situations in some Pokémon games can also present themselves as "item traps" for their specific cases.
Some of those "item traps" may lie in the puzzle spinoff games: most recently, Café Remix has some stages that practically require certain Pokémon to clear, but even then that's not enough, since the same stages practically require items to enhance the skills of the Pokémon. While some players may chalk up the possible misplay of the stages due to inexperience or not enough time spent, the fact of the matter is that somehow the items present their necessity, thereby realizing them as "item traps".
Meanwhile, in Pokémon Go, if one used a Bottle Cap for a Pokémon, the higher-level tasks of Hyper Training for the IV stat of HP feature tasks of earning Candy for a Buddy Pokémon by walking with them. If the Bottle Cap was used for a Legendary or Mythical Pokémon - and most Trainers are expected to do so - that invariably means walking 20 km for each Candy, unless they get them excited manually (which is possible but time-consuming) or use a Poffin (which is instant but... it's an item, and thus an "item trap" lies in wait). Granted, the usage case is very specific, but the "item trap" can present itself in that case.
For the main series games, if the act of giving a drink to the Saffron City guards is considered an act of compulsion, and so would be the use of items in Trainer battles involving the CPU by both sides, then these could be loosely considered "item traps" by the definition above. They aren't strictly "item traps", but they're sufficiently demanding situations that in some ways do prompt the usage of items in order to continue progress.
There are likely to be more examples of "item traps" in Pokémon games that have escaped my mind, the scope of my Pokémon gameplay experience, and the creation of this write-up, but the situations that prompt them to appear should be quite evident by the above examples. In any case, usage of items is essential for Pokémon games or otherwise, but it cannot be denied that at certain points, the usage becomes not only essential but down right needed so that one's progress doesn't come to a screeching halt.
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