Many people have HTML and/or RTF stored in a database, a dataset, or wherever. If you display this text in a Reporting Services textbox, it gets rendered as markup versus properly formatted HTML or RTF.
Currently, Reporting Services gives you no way to render "HTML as HTML" -- basically because doing so would open the door to HTML injection attacks.
Two possible workarounds have been discussed, but I've never seen them implemented:
Post-processing: After a report has been fully rendered, intercept the document and re-process it, turning the HTML (displayed as a string) into HTML which is "really" displayed.
Custom Control (2005 only): One could theoretically build a custom control in 2005 which takes the HTML/RTF, saves the rendered output as an image, and then displays the image inside the custom report item. This looks really hard as the managed GDI namespace doesn't give us anything to easily approach this sort of scenario.