Wikipedians use the Discussion pages to hash out disagreements about the content of an article. Sometimes these disagreements are pedantic, sometimes they are nitpicky, and sometimes they give you a great sense of where the main points of controversy lie within a topic discourse.
History pages give you a way to track the development of thought on a page over time. A Wikipedia article can change several times -- daily or even hourly. Every single change, though, is captured and preserved and you can go back and look at earlier versions of the page at any time. This gives you a way to see how reliably the page is being updated and corrected, and whether the discussion on the topic is still evolving.