You know how the most convincing lies are told by people who convinced themselves of it first (even if they actually don't believe it and are aware they are lying)?
Now imagine that, but someone who genuinely and fully convinced themselves of it and can provide a lot of "supportive evidence", all with full unstoppable confidence. That's what an LLM can act as, a perfect gaslighter. Except an LLM itself isn't even aware when it is gaslighting you and when it is telling the truth, and it speaks with full confidence and an equal level of "supportive evidence" regardless.
If you can't tell, it definitely matters. It is one thing to be fed info by a good liar who is a real human vs. by an LLM that can be the best liar on earth without even being aware of it.