Why not? Document functionality and possibly showing examples is great (in general I'm a big advocate for examples), but those don't preclude explaining concepts.

Some of the best documentation I've experienced covers concepts, some of the worst doesn't.

It really doesn’t take that much extra work to explain why someone should care about the existence of a function. But it makes a huge difference in how approachable the library is, and how easy it is to learn (for instance, is this even the function I’m looking for?)

I think there can be different types of documentation (tutorials, reference, explanation, how-to guides). I came across this article a while back that goes over different categories and always re-read it when I know I'm going to have to write a bunch of documentation for something and always find it helpful to really think about the type that I need to write. In regards to the poster you replied to I think Oreilly books are in the explanation category of documentation instead of the traditional reference kind found online.

The article I'm referring to: https://www.divio.com/blog/documentation/

Documentation is a moving target. Depending on your project, you often have to mix background explanations with code samples. Fields evolve. New people show up. Documentation can never really be good enough. Books provide a way of filling in certain gaps in a structured way that gives people more background. Often times, people expect documentation to do more than this. I tend to agree with you, but it's not what end users end up needing a lot of the time.