The Expanse novellas (if that's the kind of thing you mean) are all quite good, and expand on stuff that wouldn't have fit neatly into the main series' pacing/structure. The show even manages to weave them in directly.

The same thought crosses my mind that these books are probably not as good as the series of books and are more world building or cashing in on the success of the others.

If quality is your concern, there are plenty of series that have less-good main entries, or are bloated with stuff not everyone will care about, or are like 40+ books long but the same plot every time. Personally, I'd rather have a solid, streamlined series with side content available for those who really love it and want more.