You could optimise either, I don't think that's the point. It's just a convenience that more clearly expresses the intent of the code where it's used. Imagine an example where the callback to map is quite long; seeing the flatMap identifier alerts you to the fact that the callback returns arrays, even before you start reading.
You'll find equivalents in all the JS utility libraries and most functional programming language standard libraries (and languages like Ruby with functional-ish subsets), so there's a lot of evidence that people who write code in that style like to have such a function available.
You'll find equivalents in all the JS utility libraries and most functional programming language standard libraries (and languages like Ruby with functional-ish subsets), so there's a lot of evidence that people who write code in that style like to have such a function available.