Most books and magazines have pages that are taller than they are wide. Your computer screen is wider than it is tall. On pages with multiple photos or drawings, the images are small. To present book and magazine pages with the original layouts in ebooks leaves the sides of your monitor screen empty.
When “optimized” appears in a product description, it means that images have been placed on wide ebook pages that mirror the horizontal proportions of your screen. In many cases, this means that walls, floors and furniture have been cropped out of photos and illustrations to obtain images of model(s) or character(s) that fill the screen from top to bottom. In some instances, an enlarged detail is shown beside the complete figure or on the following page.
Some products only consist of an ebook presentation of the original that reproduces the publication page-by-page, as originally printed (i.e., with a vertical orientation). I call this a digital replica. Some products include an optimized version in addition to a digital replica.
Some ebooks are optimized for ebook viewing but most are not.