Moving the head helps them to differentiate something they are observing from the background behind it. If it’s at a different distance than the cluttered background, moving their head left and right will cause the thing they’re observing to move relative to the background. Also if something were in between them and what they are trying to observe, such as spanish moss or some vines, then moving their head would enable them to see all of the object, even though some of it would be occluded at any given time. If you think about it, you see people doing this all the time when trying to see something under conditions where there’s clutter in the field of view!