They do this for two reasons. To build up the internal pagerank of their blog, to lead both visitors and search engine robots to old content.
Search engine robots spider a site by following the links. Linking to other content, especially old content, on your blog ensures that everything gets spidered by the robot.
Additionally, you get new visitors to your blog everyday. Very few will go back and read what you've got in your archives. So in order to draw attention to your old posts, you have to link to them in new posts that you write.
Some blogger have a "Best Of" list of posts in their side bars. Some bloggers, including myself, write follow up posts to previous articles. For example, you write an article about the importance of widgets in the blogosphere. Some days, weeks, months later you would then write a follow up post about how other bloggers are putting the widgets to good use on their blog.
I don't know if there is a plugin or not that will do this. I think there is a WordPress plugin that will link certain keywords to your archive pages but I'm not 100% sure. You would have to search the
WordPress Plugin Repository to find out.
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