The reason it’s recommended is that it’s useful for detecting when an email you receive is already in your mailbox, so that you don’t accumulate duplicates. Otherwise one would have to compare the complete email, which probably no MUA does. Another reason is that replies can include a reference to the original message, so that it can be properly threaded by MUAs.

So these are mostly quality-of-life reasons, it’s not a reason to reject an email.