I know on reddit since basically the very beginning there has been a market for accounts with authentic but anodyne histories. It ends up being easier to make them yourself and then just occasionally use them for whatever guerilla marketing, astroturf campaign, or propaganda operation is your actual goal. But still until recently you pretty much had to pay people to sit and post on social media on a bunch of accounts to generate these histories.

This use was one of the first things that occurred to me when LLMs started getting genuinely good at summarizing texts and conversations. And I assume a fair bit of this has always happened on HN too. I've never moderated here obviously so I have no first hand insight but the social conventions here are uniquely ripe for it and it has a disproportionate influence on society through the dominance of the tech industry, making it a good target.