There are a LOT of items to find and pick up in Baldur's Gate 3, and not all of them are as clear on what you do with them as others. Some items will clearly state what to do with them, some are explained through side quests, and some are only mentioned in passing dialogue. Of the latter, Soul Coins can be one of the most confusing items in Baldur's Gate 3.
What are Soul Coins used for?
Soul Coins aren't actually a form of currency in Baldur's Gate 3. It's not like you can exactly spend them anywhere. They're used to power an Infernal Engine and are commonly used in the Hells. However, you've got somebody on your team with an Infernal Engine, Karlach. You can give Karlach one of these Soul Coins to 'power' her up which gives her 1-4 extra fire damage whenever she's raging or while she has 25% health or less, and the condition lasts until a long rest.
It's worth noting that Soul Coins are rare in the world of Baldur's Gate 3, especially since they're not actually supposed to be on this plane of existence. Use them as wisely as possible, maybe using them only for big fights that involve a lot of enemies or massive story beats such as boss fights. Don't just waste them on side quests or on casual encounters.
You can also give Karlach a certain amount of Soul Coins then interact with her to get a unique dialogue moment where she basically acts like she's high off the Soul Coins, which might be up there with one of the funniest scenes in Baldur's Gate 3, which is saying a whole lot.