My personal hunch is that, at least for me, the benefit stems from constantly-changing scenery. I'm not actually paying attention to it most of the time, but little bits catch me here and there, and in any case I'm aware of it. This passive, non-urgent sensory stimulation doesn't make me forget what I'm thinking about, but constantly interrupts my inward fixation just enough that I don't get stuck on one thought -- so new ones have a crack to wedge themselves in through.

It could boil down to an instinctive awareness that I'm not in a place I know is absolutely safe, so I have to be ready to accept and react to external stimuli at any moment, even if I'm not currently feeling threatened; and creative thought, in this case, hijacks the "external" tag.

But yeah, science would be nice too.