In the template editor dialog, there is a [] button on every field. Its tooltip is "Fall back to plain wikitext". I always saw it, but never really understood what it is. I didn't even realize it's a button until I read what User:Robin Strohmeyer (WMDE) wrote about it on the page :meta:WMDE Technical Wishes/Templates. And even after reading that, I don't understand what does it do. I see that it changes the text fields to a monospaced font, but I don't see any other effects. If I understand correctly, the values in these input box are plain wikitext in any case, although I might be wrong.
So, if it doesn't do anything, perhaps it should really be removed, as the proposal on that page says.
If it does something, I don't understand what it is, and it should be documented here. I can't see that it's mentioned in the User guide, but maybe I am missing it.