In my experience LLMs offer two advantages over private thinking:

1) They have access to a vast array of extremely well indexed knowledge and can tell me about things that I'd never have found before.

2) They are able to respond instantly and engagingly, while working on any topic, which helps fight fatigue, at least for me. I do not know how universal this effect is, but using them often means that I can focus for longer. I can also make them do drudgery, like refactoring 500 functions in mostly the same way that is just a little bit too complicated for deterministic tools to do, which also helps with fatigue.

Ideally, they'd also give you a more unique perspective or push-back when appropriate, but they are yes-men too much right now for that to be the case.

Lastly, I am not arguing to not do private thinking too. My argument is that LLM-involved thinking is useful as its own thing.