The idea is that everyone should read all posts on a topic, not just the ones they agree with. You can tell which post a reply is replying to. It’s virtually impossible to read replies in Facebook as it takes nearly two clicks per post to be able to read them. In discourse you just keep scrolling.
If you have millions of users, as some discourse forums do, it’s best that they not all participate in a single topic.
Yes there are some plugins that will let you vote on things.
Edit: see also The Universal Rules of Civilized Discourse