Been meaning to ask… how do you think items work? Berries are obvious, but items that give type bonuses are harder to explain, and then there’s items like EXP Share (how do they get experience without fighting) or Amulet Coin (where does the money come from)?
Well they definitely don’t all work in the same way, so realistically this is not going to be an exhaustive answer, but let’s try.
To answer the question with another question: why are Pokémon allowed to use items in battle at all? Berries, I suppose, you can excuse, since wild Pokémon do it and they’re just natural supplements and snacks, and plausibly the same holds for Herbs (of the White, Mental and Power varieties), Leftovers and manufactured foods like Lava Cookies, but how is it fair to let a Pokémon carry a tool that measurably makes its attacks more powerful? Why not give them Mediaeval plate armour and maces at that point? Why has no one in any known Pokémon League cared to draw a line somewhere on the saner side of spectacles that amplify magic?
Answer: because the physical items themselves are somehow not where the benefit is coming from.
The type-boost series – Silk Scarves, Charcoal, Miracle Seeds, Nevermeltice and so on – are essentially totemic. They’re rare and special, they may have unusual properties unrelated to battle, and in some way they remind a Pokémon of the source of its power, whatever that is. They don’t actually do anything else, but the belief that they are special is enough to strengthen attacks of the associated element. This belief, and the associated effect, may be stronger for particularly rare items that were obtained by the Pokémon’s own efforts, especially at personal cost. The same probably holds for most items that appear to be somehow symbolic of their effect but don’t seem to offer any mechanism for actually achieving it, like the Grip Claw, Binding Band, Heat Rock (etc.), Shell Bell, or Quick Claw. Things like the Choice items, Expert Belt, or just conceivably the Assault Vest could be imagined to work similarly, but evoking a fighting style or technique rather than a source of power.
Items that are notionally harmful to the user like Toxic Orbs, Sticky Barbs or Ring Targets might be allowed on the grounds that you can have whatever handicap you want, and getting the opponent to take one from you requires enough cleverness that you deserve the payoff. No one seems to have thought of invoking this principle in the case of a live hand grenade, but I suppose there must be a reason Fling is a Dark-type attack.
Some items really are probably magic, which is a problem because the Pokémon universe doesn’t have general rules for how its magic works, and if you were so inclined you could handwave almost all items as “magic” without having to really think about anything, so this should probably be a last resort explanation. Amulet Coins are koban, the same type of gold coin worn by Meowth on their foreheads. However exactly they work, it probably has something to do with Meowth’s signature move, Pay Day, and Meowth’s associations with money, good luck and prosperity. If Pay Day is allowed, then by extension Amulet Coins have to be as well. A lot of the objects associated with legendary Pokémon, like Soul Dew or the Adamant, Lustrous and Griseous Orbs, are probably literally magic, but so closely associated with the Pokémon that use them, so sacred (and in any case so ridiculously rare), that no-one has thought to regulate their use. Mega Stones and Z-Crystals add another dimension to the question. Those items too really are magical (probably, anyway), but using them correctly requires a strong bond with your Pokémon, which gives you a chance to claim that their wielders deserve their advantages. It’s definitely not fair that some trainers get to help their Pokémon fight with powerful magic, but it does reinforce the values and strengthen the adherents of the dominant ideology, which is almost as good as long as you’re part of the club.
The Exp. Share is surprisingly forthcoming with answers, but not all of them make sense. It appears to be a sort of headset, with blinking lights, electrodes and headphones. It looks like how a mad inventor might design a virtual reality headset, and its function is to transfer to the user “experience” – that is, knowledge of circumstances, situations, actions, events, and consequences. You might conjecture that it allows any Pokémon with a head to observe battles in real-time from a first-person perspective, and thus pick up skills employed by the target during the battle. It’s weird that humans don’t use these to learn all kinds of skills, though, and it’s very weird that the target doesn’t need a corresponding headset. Can the thing just siphon thoughts and sensations at a distance? Could you use it to spy on people? Probably it has a whole bunch of restrictions that we won’t be able to nail down 100% (since we can’t test it outside the narrow circumstances given to us by the games) – maybe the user needs to be familiar/friendly with the target, or the target is aware and has to “let them in,” or maybe it can only transfer adrenaline-fuelled thoughts or certain reflexive behaviours. Or maybe some Pokémon Professor looking for a nifty training gadget invented the most powerful espionage tool in the world and no one has figured it out yet.
And a Weakness Policy is obviously the signed text of a literal Faustian pact that grants terrible power at a terrible price.