It's just manners, isn't it? Think of it as a conversation around a table, which might well go into diversions or digressions at times but if someone has actually asked a question or sought support, then it's rude to derail that.
On gransnet, let's say someone posts a problem, and someone else comes along - mid-conversation - and says they have a similar problem and goes on to ask about their problem, rather than using their experience to illuminate the OP. People start answering or commenting on that 'new' problem, and the OP's concerns get lost or confused in the subsequent discussion.
That's bad manners. The 'similar problem' poster should have started a new thread.
In the same way, a poster who uses a thread to snipe at another poster or bring a personal grievance, is being rude.