I assume given the nature of HN that a lot of people use it as a training ground or an experiment, they just do it because they can. Some people may also believe that using an LLM is a more efficient and productive way of interacting with the site, and that it generates higher quality content than they themselves could. I suspect at least one person has tried training an LLM on their own content and posting through it. Maybe they want to farm karma, although I don't know why they would.