The main case I can think of is wanting some forum functionality. Perhaps you want to allow your users to be able to write in markdown. This would provide an extra layer of protection as you could take the HTML generated from the markdown and further lock it down to only an allowed set of elements like `h1`. Just in case someone tried some of the markdown escape hatches that you didn't expect.
I think this might be the answer. There's no point to it by itself (either you separate data and code or you don't and let the user do anything to your page), but if you're already using a sanitiser and you can't use `textContent` because (such as with Markdown) there'll be HTML tags in the output, then this could be extra hardening. Thanks!
I wouldn't trust myself to always remember to sanitize it, and in a company with more than one person, it becomes impossible to ensure it is properly handled.
The main case I can think of is wanting some forum functionality. Perhaps you want to allow your users to be able to write in markdown. This would provide an extra layer of protection as you could take the HTML generated from the markdown and further lock it down to only an allowed set of elements like `h1`. Just in case someone tried some of the markdown escape hatches that you didn't expect.