People can discuss there as well. Discussion can make people understand things better, or gives people a chance to tell their views. In Wikipedia the talk pages are for that, but in some wikis, the article and the discussion are in the same page. Wikis can be used for different things and all wikis don't follow the same rules for using them. For example, the purpose of Wikipedia is to write articles, which form an encyclopedia. That's why in Wikipedia, people don't want to hear general discussion that doesn't help in writing articles.