The plugin is especially handy if you use Bootstrap or Foundation framework on your website: in this case you can painlessly make content of the document in CKEditor or in TinyMCE responsive and correctly marked up. Thanks to fine-tuning you can specify which components of Bootstrap/Foundation you want to apply. The add-on supports downloading from the official CDN as well as from your local installations of the framework.
If you use your own CSS framework or custom styles, you can manually specify a set of file to include. That is, you can adapt the edited document to the style of your website. It is much easier to edit pages when you see them exactly in the same fonts, sizes and colors as your main theme. This allows you to stop using the preview functions every minute, which is basically the main advantage of WYSIWYG by design.