See T418352 for discussion that prompted this.
CodeMirror is first and foremost a syntax highlighter. But it is in fact a proper editor, and the highlighting itself doesn't strictly need to be visible for other fancy features to work. For some, the parser will need to be there to do its job, e.g., code folding and bracket matching. Other features like line numbering or highlighting the active line don't need the parser at all.
After discussion, we've decided to offer the ability to disable highlighting entirely as a theme (T163533). That way, all other features will still work.