Personally, I’ve found that LLMs are best as discussion partners, to put it in the broadest terms possible. They do well for things you would use a human discussion partner for IRL.

• “I’ve written this thing. Criticize it as if you were the recipient/judge of that thing. How could it be improved?” (Then address its criticisms in your thing… it’s surprisingly good at revealing ways to make your “thing” better, in my experience)
• “I have this personal problem.” (Tell it to keep responses short. Have a natural conversation with it. This is best done spoken out loud if you are using ChatGPT; prevents you from overthinking responses, and forces you to keep the conversation moving. Takes fifteen minutes or more but you will end up with some good advice related to your situation nearly every time. I’ve used this to work out several things internally much better than just thinking on my own. A therapist would be better, but this is surprisingly good.)
• I’ve also had it be useful for various reasons to tell it to play a character as I describe, and then speak to the character in a pretend scenario to work out something related. Use your imagination for how this might be helpful to you. In this case, tell it to not ask you so many questions, and to only ask questions when the character would truly want to ask a question. Helps keep it more normal; otherwise (in the case of ChatGPT which I’m most familiar with) it will always end every response with a question. Often that’s useful, like in the previous example, but in this case it is not.
• etc.

For anything but criticism of something written, I find that the “spoken conversation” features are most useful. I use it a lot in the car during my commute.

For what it’s worth, in case this makes it sound like I’m a writer and my examples are only writing-related, I’m actually not a writer. I’m a software engineer. The first example can apply to writing an application or a proposal or whatever. Second is basically just therapy. Third is more abstract, and often about indirect self-improvement. There are plenty more things that are good for discussion partners, though. I’m sure anyone reading can come up with a few themselves.