Sometimes adding images can look a little weird (i.e. you add a back cover image that doesn't have quite the same proportions as the front cover image and it looks awkward when they scroll), so it'd be nice to be able to see that and other formatting issues prior to final submission.
Text-heavy areas like the box text are hard to look at in their text fields on the submission form, and it'd be nice to see how the formatting looks on the actual page (plus catch any typos).