The author doesn’t gain anything, it’s entirely to help the reader understand their own readiness for the material. It’s not rude, it’s straightforward and matter of fact.

If you don’t meet the prerequisites, you should avoid the material. Not because the author has stated the obvious, but because you’re not prepared for it. If you’d like you can interpret it as rudeness, but really it’s intended to help people save time instead of waste it.

The first thing I do when I open a new textbook is flip to the preface material to see what the prerequisites are and how firm they are. Sometimes “passing familiarity” is enough, sometimes “mathematical maturity” is enough. But there are many treatments where that is not the case, and it’s better for everyone who’s serious about the material to be upfront about it.