... but if you don't supply one of course Let's Encrypt won't notify you about anything.

So if you aren't paying attention you may get blind-sided by any future change, particularly if your use case is weird e.g. you can only pass http-01 by HTTP 301 redirecting to a machine with a completely different hostname, works today, could get outlawed as dangerous one day and they'd have the records to show you're going to be affected, but no way to automatically warn you.