I'll drop in from the sidelines, with the massive track record of having written one (1) book.

The prestige probably isn't what you'd expect. Having an ISBN to one's name carries ~zero weight for the people that actually matter for your career (it may mildly impress some future coworkers in a decade's time, though). The real value of having written a book is that then you have written a book.

Having a publisher carries one extra benefit that was merely implied in the post: you get assigned a professional editor. If you're lucky (I was), the editor has a really good understanding of how to wield language and the lessons you get from the editing process are going to far outweigh any direct financial benefits. When I wrote mine, I had been doing freelance writing for a large IT magazine for nearly ten years - and as a direct result had been taught how to use written language as a weapon by a good number of old-guard journalists. The year I spent on the book project taught me a LOT more still, because I was assigned an editor who herself was studying (in a university) to become a language teacher.

The skills I picked up from that process are with me to this day, and ironically have been the single most valuable asset I have as an engineer and/or engineering leader. Being able to write well to a varying audience is a superpower. You also learn to appreciate professional authors, because what they do is decidedly not easy.

In the end my book sold well enough to earn out its advance, so I guess it was a non-failure for the publisher as well. I also picked up a lesson for all aspiring authors:

Writing a book is easy. You sit down by the keyboard, slit your wrists and pour it all out.