If you’ve made your way through Pokémon Sword and Shield, you’ve likely obtained at least one Bottle Cap. Don’t sell it.
The Digging Duo can dig them up, you can grab them from Max Raid battles, and even your high-leveled Pokémon with the Pickup ability can find some laying about.
Bottle Caps are sorted into the “treasures” section of your bag, a section mostly littered by expensive items — like Pearls and Nuggets — that players can sell to NPCs for money. This can bait some folks into selling one of the most useful items in the game.
The rare and elusive Bottle Cap can be used to max out one of your Pokémon’s IVs. IVs, or Individual Values, are stats your Pokémon are “born” with and under normal circumstances cannot be changed. However, once you beat the game and unlock the Battle Tower, any level 100 Pokémon can get their IVs maxed with the use of a Bottle Cap, through Hyper Training. An NPC in the Battle Tower will max the IV of one stat of one of your Pokémon in exchange for one Bottle Cap.
Maxing out IVs isn’t really necessary for folks just looking to complete the game, but if you want to take on the deeper levels of the Battle Tower, or start making some Pokémon for competitive battling, it’s a must have.