I think it's probably the same motivation as those people who post "I asked [chatbot] and it said:" comments. Granted those are more courteous, but I think both are motivated by people who are too easily impressed by the present capabilities of the chatbots and genuinely think they're making helpful contributions by inflicting the chatbot comments onto the rest of us.
Gain reputation, use the account to manipulate us at a later time. HN is a high value target for the same regimes that use Reddit and other social media platforms to manipulate the US and Europe.
We know nation states try to compromise social media platforms and Github; that isn't conspiracy, it's fact. Now whether this particular set of users serve that purpose, I cannot know. But why else would someone use a bot on this site, other than to gain reputation for the purpose of manipulation of some sort?
It is a certainty that you do interact with malicious actors representing hostile nation states on any large platform - HN is well beyond the size where it's an issue. I hope the HN team has some tools to detect and counter it, but given their small team size they probably can't do much.
I'd imagine there are monetary incentives to having many accounts able to boost you and keep your post on the front page. Doing that with brand new accounts who have no comments and no karma is probably harder. In the cases listed, all the comments were heavily downvoted but there are other bots out there who have a positive karma.