Text can be edited directly on page, including all styles that theme designer has crafted for Your website. All text formatting (including custom fonts), images, videos are rendered in a same way as page for Your visitor. Thus You can be assured that Your edits are consistent and robust.
Arrange content by splitting it into arbitrary blocks and columns. Create layout like in newspaper, classic way in one column, or combine both to make reading website pleasure.
Styles designed specifically for a theme can be used within editor. Theme designer can easily configure which style should be used for which part of website, with skills he already know. For web editor it's matter of choosing it from the list to use those styles.