It's just especially obvious in Space Marine books. We're getting battles just because the book should have fighting in it, rather than because they advance the plot or affect the people involved in a meaningful way... Well, they might, but we never get to KNOW the people so what's the point.
The problem with BL is that they don't seem to realize that books should be about CHARACTERS. People, not just fighting. You'd think it would be obvious with long-running series like Cain and Gotrek & Felix... Much like those, the good Marine books manage to get plot advancement, character interaction and/or development in during the fight scenes. Obvious example from James Swallow's Blood Angel books, Dante and Gabriel Seth fighting side by side.
Mediocre ones - and I'm counting Dark Imperium among those - only manage it in between. Guy Haley makes a brave attempt at it, but he's stuck with an ensemble cast that needs screen time on everyone. The apparently mandatory number of battle scenes largely turns into meaningless interpunction that can be skipped without missing anything. The book is (IMO) at it's best during the character focus scenes - Calgar's musings, a squad of Primaris marines just chillaxing and highlighting the marines of various legions coming together, Guilliman's interactions with the Cawl Inferior, Typhus telling Mortarion what's what.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.