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.