Why would you need to set up a postgres db...? Unions are a fundamental set theoretic operation that are applicable to all set-like collections. You may as well say "an in memory hash map / list is absurdly inefficient compared to a relational db." Is it efficient, to you, to spin up a postgres instance to hold a dozen key value pairs?

Sets are super useful for all sorts of stuff where you want the datatype to guarantee there is only one instance of a value in the collection. UI components where you are incrementally adding things to a selection is a great example.

It's a completely standard feature in many stdlibs and has nothing to with turning JS into a database.