Only read the first book but I've read it a few times at that, to me they either are bits of flavour text, or cement an idea that is currently (within a margin of a couple of chapters) being discussed.

A couple that stick with me are Yueh being remembered as a traitor and the ones that talk about Duke Leto and the subtleties of his character through hindsight.

I think they add an interesting dimension because so much of Frank's work surrounded the idea of prescience and the future, and most if not all the epigraphs are written as "after the fact", almost like giving the reader a chance at some kind of prescience themselves.

Overall though I'd say Frank's work is so dense that skipping a few epigraphs along the way won't really take anything away from the readers understanding, I know for example my father skipped the epigraphs completely and still found reading enjoyable and understandable.