After reading some of these comments I still wonder what the concrete use cases are. What are these websites that allow users to paste in HTML, and why? Is that

After reading some of these comments I still wonder what the concrete use cases are. What are these websites that allow users to paste in HTML, and why? Is that even a good idea? I can understand when it's developer tools like jsFiddle and the like, but when should a normal consumer website be hosting untrusted code in the frontend?