They're extremely useful in capturing the "business rules" for a given domain, particularly when it's a restricted domain: if something can only be `A | B | C`,

They're extremely useful in capturing the "business rules" for a given domain, particularly when it's a restricted domain: if something can only be `A | B | C`, being able to make that explicit, have it checked exhaustively everywhere by the compiler, and be able to dispatch on it easily using `match` is really powerful. That's because it comes with a statically-tracked "tag" (so: sometimes called a "tagged union") that the compiler can use to check you. You can write the same thing with a `union` in C or C++, with an `enum` to tag it, but you're doing that by hand rather than having it built in. Also: Where a struct or class's relationship over its members is the product of all of the types which make it up, a tagged union/enum is the sum of them. A common example: instead of a bunch of optional fields on a single class to model the different possible combinations, modeling something as only the legal set of combinations of fields. I recommend Scott Wlaschin's 'designing with types' series (uses F♯, but applies to any languages with 'sum types') for a great intro and then deep dive on the ideas here: https://fsharpforfunandprofit.com/series/designing-with-types/ https://fsharpforfunandprofit.com/series/designing-with-type...