Probably the bot owners are conducting tests to see what exactly they can do with these newly emerging AI techs and how far they can push the tech for their own ends.

Although I'm not sure exactly how them programming their bots to Necro old GOG forum threads helps them to do that.

Perhaps they are testing how many real GOG forum members become fooled into thinking that their AI-written bot posts were real people? In other words, they could be gauging to see how realistic their AI bots currently come across as being...which for the moment, isn't very realistic...but that could change in the coming months and years.

I'm fearful of a future where you can't distinguish between human and computer no more and between true and fake. A future where computers can pass the Turing Test seems to come close with AI. If the AI gives such realistic answers, that when talking to them you can't tell no more if you're talking to a computer or an AI, that's when the Turing Test has been passed. A frightful future.