I agree with everything else you said but I totally disagree that functions like that are "niche". It's true that many codebases are written such that they have

I agree with everything else you said but I totally disagree that functions like that are "niche". It's true that many codebases are written such that they have few of them, but that's "accidental" not incidental. Almost nothing is pure math, but almost everything is data transformation that is deterministic given some set of parameters and dependency configuration. And mocking out external state to control the behavior of the thing being tested is great. But the point is not to say "and then this method of that dependency is called with this input", it's to control the behavior of the thing being tested, by controlling what its dependencies return.