This is called "having a conversation". In flat discussions you can't bicker endlessly because a large portion of the posts are just replying to the OP. There isn't as much back-and-forth or progression of the discussion. Discussions tend to get only a few levels deep.
The only time anybody has changed my mind has been on nested forums, like this one. It is because we had to go back and forth three or four times to arrive at a new understanding. Nesting let us track that discussion.
Maybe it's a personality flaw, but I have had far fewer enlightening experiences on flat forums!