One of the neat features of discourse is linking topics together. The general idea is that too-long topics are a pain to read, but it’s important to maintain the context that came before.
If you want to make a related-but-not-really-on-topic comment in a thread, please click the link button (resembles a chain) and then “+ new topic” button. The old conversation will be referenced, but it won’t pull all the history along with it. This allows the new topic to move in its own direction instead of making the original topic confusing to read.
If you have something to add to an old topic (One that presents a warning when you click the reply button), do the same.
If you don’t have anything to add, i.e. just want to make a random comment, don’t do it.