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.

It may well be different in NL, I’m not going to argue with that. But ‘ontwikkelaar’ is literally a different word in a different language - no surprise that it may have different semantics to the closest word in English!

This is not true at all, except maybe in very specific places (e.g. some jurisdictions do not allow you to call yourself an engineer without a specific qualification).

Software Engineer and Developer are essentially interchangeable terms and largely a matter of taste.