The original topic is quite old and I didn’t remember all the details, but re-reading it now I believe @Moin is right in identifying the key point.
Back then, our use case on the support forum was to avoid long, sprawling discussions in support categories. We wanted to keep topics focused and concise, no more than 100 replies (ideally less then 50), to encourage users to actually read through the information instead of skipping to the end. However, we didn’t want to use a time-based auto-close, because even months/years later a topic might still be relevant due to our software’s extremely slow release cycle (basically only patches and security updates).
At the same time, we had non-support categories (like politics, economics, and general discussion) where we didn’t want any artificial limits. These were meant to be more community-driven, with freer conversation and minimal moderator intervention.
So the goal was to be able to apply post count limits selectively, category by category, instead of globally. That way we could keep support topics lean, but allow open-ended discussions elsewhere (without getting into mega-topics of course).