It's not a rule, but it is a general etiquette on forums.
While we may be all literate, it is human tendency to better process information in chunks of distinct thought. This is evident in many effective/efficient systems of social organization. I think the idea of and reasons for starting a new post does fit with this thinking. While it may be on the same idea as what someone else was discussing, it may often be more useful to break out a separate topic. That way the flow of thought that exists in one topic doesn't become split if there's a separate (although related) thread to discuss.
Now, if what you have to add is more in line with the specific discussion, I think it's more welcome on the same thread. But if it's talking about your own topic (even if related), but not actually about the original poster's topic/thread, then it will usually be seen as rude. Forums often call it "thread/topic hijacking".
To specifically address your question as to why have a million posts on one topic... a few reasons. 1) Each topic is really intended to be about discussing the original poster's original post (or tangents they flow with). So, in a way, original posters (commonly called OPs), have priority for their own topic. 2) In real life, many people have conversations on the same topic. Separate conversations allow for more variety in what actually gets discussed, and what tangents they may explore through discussion. A mono discussion would offer less variety. But now I'm just getting philosophical.
So in short, not a hard fast rule, but it is generally good forum etiquette. Sometimes it can be a fine line. But I think the way to gauge it is, if you're talking about yourself or your own issue (and not to provide an example FOR the OP), there's a chance it may be better as a separate topic.
Cheers,
-Alex
