Data > Convert can be useful for creating a new document in plain text or in rich text.
A neat touch is that the metadata (including URL, Path, Creation Date) of the original document are retained in the converted document.
An HTML page or a Web Archive, for example, can be converted to rich text, including images and working hyperlinks. Or a plain text conversion might be chosen to get rid of pesky advertisements.
This can also be useful for extracting the text of a PDF document.
Just select a document for which text conversion would make sense, then select Data > Convert > (text option).