It's useful for ideation: suggesting ideas and concepts that you might not think of. A bit like a conceptual thesaurus. But it doesn't replace the hard work of thinking for yourself.
a) No / little data: Whenever you are starting to think about a subject, you can ask it to give you a structure / categories.
b) Existing data: What I do very often is to give it a lot of "raw data" like unstructured thoughts or an unstructured article, then I ask him to find suitable top categories.
I see, I don't want to shame this kind of use. It's kind of almost like talking about something briefly with an educated person.
Until it's not.
I'm not the type who'd say "don't use AI". Use whatever works. Myself I became really fascinated by transformer LLMs / GPTs in winter 2019, then again when ChatGPT was published and a good few months after that.
It's just that my interest&enthusiasm has almosted vanished by now. Surely it will reemerge at some point.