> Isn't this what parenthesizes are meant for?

Parentheses add emphasis to a sentence or statement. Normally the use of it allows the sentence to be complete with or without it.

Em dashes may also add or increase emphasis but are normally treated as an aside. Think of it as a comment by the author to inject themselves, sometimes in ways which do not form a complete sentence.

For example: When you read this sentence (in your mind) it should feel complete and correct. Perhaps you read in your own voice — something I don’t normally do — or without one at all.

> I don't understand what the issue even is here, and the RFC also doesn't clearly outline it.

The issue is written there but may not make sense unless you know someone who stylistically writes with high-than-average em dash usage. I, for example, get inquiries and comments at work from employees who ask what LLM model I used for “generating these reports” because of the presence of em dashes. They do not believe me when I say not a single word was written by LLMs because, “there’s an em dash. Only LLMs use em dashes!” This is categorically untrue and erodes the authenticity of work from people because of the correlation.

Their aim is to implement a new Unicode character which programs like text editors could inject when a person types an em dash. It attributes to a human being behind the document, typing characters out individually. Actions like copy-pasting text in bulk wouldn’t replace em dashes since it can’t attribute a human as writing it out.