Being on-topic is easy for an LLM to achieve. Being useful is harder, and that definitely hasn't been achieved in this case.
So it's just time-wasting slop.
Probably. Same purpose(s) as all other spambots, just using new tools: Either they will edit their posts later to insert spam links, or they're just vandalizing for the sake of it.
That would indeed be putting AI to much better use.
This part has harmful side-effects. Software community forums everywhere are shrivelling up, and I bet that one big reason is because people now talk to bots more, and talk to each other less.
Sure, this makes answers more readily accessible, but the costs are:
The loss of motivation to learn how to ask questions well.
The loss of opportunities to share our knowledge with others directly.
The loss of human networking in general.
These activities have intrinsic social value beyond just acquiring information.