How the brain operates in the absence of other stimuli. It starts brainstorming stuff by itself! The incubation effect occurs.
Shower thoughts? Bed thoughts? That is the origin of them. Modern life has reduced these through, for instance, social media. A good method is to write down creative thoughts made in the default mode network as soon as possible and elaborate on them later (just like my incremental writing habits); however, maybe I'm projecting my habits to you?
Saying that a lack of default mode network is bad.... for a writer it might be bad. But for other people, you cannot really generalize it. Because they might have stuff to do or like music more.