Currently, I use it mostly for formatting. It's quite easy to generate a nice looking document for Overleaf or something like that.
Then, I have a workflow to make 'monster tear sheets'--basically a condensed spreadsheet with stats for the monsters I'll use in a session, so I don't need to reference the book. I had a version for this for 5e that I wrote by hand, pulling from the SRD. I used AI to clean it up substantially and make it easier to use.
I played around with AI as a campaign setting assistant. They were useful in some ways. For example, I'll often tie things in my world to real history. I can describe a vague idea, look up historic parallels, read into those further, and then develop that.
I imagine the state of things is much stronger now than when I did it. I've had an idea to try again seriously, but I'm not running anything actively and haven't had the time. I think it could quite sophisticated, because you could store your random tables, longer-term memory files (npcs, locations, campaign history &c), other details about style preferences.
One thing that is lacking here is the ability of AI to process pdfs. Ideally, I would want to be able to store modules in the same directory for easy reference, and I'd want the AI to be able to read and reference material from those. It can, but it is token intensive and more error prone than reading markdown files. PDFs are a hard problem, but I think it would be cool if some creators started offering less structured markdown files along with the pdfs. I don't think it would be too big a challenge because the formatting is minimal.