Sometimes people indeed just need validation and it helps them a lot, in that case LLMs can work. Alternatively, I assume some people just put the whole situation into words and that alone helps.
But if someone needs something else, they can be straight up dangerous.
They have world knowledge and are capable of explaining things and doing web searches. That's enough to help. I mean, sometimes people just need answers to questions.
In one way it's potentially worse than talking to yourself. Some part of you might recognize that you need to talk to someone other than yourself; an LLM might make you feel like you've done that, while reinforcing whatever you think rather than breaking you out of patterns.
Also, LLMs can have more resources and do some "creative" enabling of a person stuck in a loop, so if you are thinking dangerous things but lack the wherewithal to put them into action, an LLM could make you more dangerous (to yourself or to others).