At first glance you might think Recent Posts and New Posts will take you to the same place. However there is a subtle difference.
New posts will show you the last 200 new posts in the forum that are NEW TO YOU. If you have seen the post before it will not show up in New Posts. So if you have opened a thread, the forum assumes you have read all the posts within and will not show you that thread again until there is a new post added to it.
New posts is a great way to catch up if you've been away from the forum for a few days and want to see everything you've missed. Read away and every time you reload the New Posts page it will show the latest 200 posts that you have not seen and hide all those you have read.
Clicking recent posts simply returns the latest 200 posts whether you have seen them before or not. Recent posts is the best way to monitor the forum when you are visiting very regularly or are constantly online.