Although the Compose bar is designed to save you from writing HTML, it is perfectly fine to type HTML tags right in there; you don't have to switch to the Edit HTML mode. If you do switch to Edit HTML mode, you'll notice the HTML code stays intact. But when you switch back to Compose, the HTML code will be displayed as if it were in a browser.
One reason you might want to write your own HTML (apart from being a control freak) is that the Compose bar only offers a limited set of formatting options. But you can type any HTML you want manually. My bestselling HTML for the World Wide Web, Fifth Edition, with XHTML and CSS: Visual QuickStart Guide is a good resource, if I do say so myself. :)