Understanding mind-wandering could shed light on mental illness
If you think the mind grinds to a halt when you're doing nothing, think again. Spontaneous thought processes -- including mind-wandering, but also creative thinking and dreaming -- arise when thoughts are relatively free from deliberate and automatic constraints. Mind-wandering is not far from creative thinking. Ref. Source 3p.
Maybe that is why creative thinkers have a knack for thinking outside the box because they do not constrain themselves or put barriers to thought.