As anyone whose worked with the visual editor knows, the way content appears within it often bears little resemblance to how it will actually look on the page/post. The appearance of fonts and other things may not be the same, In other words, what you see isn't really what you get. This can sometimes be a big hassle, requiring you to do multiple previews to get the content to look right.
I wanted a way to fix this, to make content in the visual editor duplicate the way it looks on the actual pages/posts. I was doing so by editing the styles in wp-includes/js/tinymce/themes/advanced/skins/wp_theme/content.css, but, of course, that meant that I would have to redo the edits every time I upgraded WordPress.
Happily, this plugin gives me another option. With it, I was able to change the font face, size, color, and line height, as well as image padding and some other things to match what shows on the page/post.
I think it would be great if the author of this plugin expanded its functionality and turned it into a way for people to fully control how the visual editor displays content so that they can do what I did.