I guess if you're just reading for content, it would be helpful, especially
for those who need to increase the text size for visibility. In any other
view, if you increase the Zoom ratio, eventually a document with reasonable
margins will run off the screen, but Reading Layout/Full Screen Reading view
allows you to increase the font size without increasing the width of the
text line. I would guess it is intended for e-books and mimics the behavior
of dedicated e-book readers such as Kindle. It would probably also be
helpful for just ordinary proofreading. Sometimes I overlook missing
punctuation because it falls at the end of a line; with both nonprinting
characters and text boundaries displayed, it's often hard to tell whether a
period or comma is actually there or not (unless I increase the Zoom
dramatically). So I would guess it has its uses for some people.