iframes are a Microsoft creation and not a part of the official HTML spec even though most browsers support them. there are some browsers out there that don't.
Firework builder plugins adds the plugin page to the main page on the server side, before it is passed to the client. As far as the browser is concerned, the plugin page is a part of the main page, there is no relying on a tag (iframe) that may not be supported in the browser the client is using.
in short, there are some browsers that won't support iframe. Framework Builder works at the server level, so the page will work as expected regardless of the browser