RTF supports only basic formatting and produces bulky files. HTML opens in any browser and can be published on the web as-is. RTF is at home in basic formatted text that must open anywhere; HTML is the usual choice for web pages and emails.
The HTML is made inert: scripts, event handlers and forms are removed, remote pictures become plain links and nothing in it is fetched when you open it. Headings (styles named Heading, or a bold paragraph set much larger than the text), lists, tables and PNG or JPEG pictures are read; fonts, colours, headers, footers and footnotes are dropped.