TLDR: Communication from a human is not only the material being communicated, it's also a signal that the human understands the message they are sending you. When you pass that message via an intermediary (regardless of whether the intermediary is a human or an LLM), the receiver is never sure that what they received is what you intended.
It's doubly worse when you review it, notice a few additions and think "Great - that makes me sound smart and knowledgeable about this topic", because the receiver now has an inaccurate view about your skill.
At any rate, using an LLM to construct a message for someone else to read is basically sending the wrong signals about your skill levels.
(This probabyl explains why so many people are so ready to use an LLM to write the material - in their minds, it makes them sound smarter than they actually are, but as we've seen from people on this forum vigorously defending the practice with "pangram is wrong" and similar nonsense, they themselves are not skilled enough to recognise slop)