Many search engines have stated time and time again that they crawl pages they can get to easily. Basically, if it takes more than three steps to get to a page, it won’t get indexed as easily or or re-indexed as often. By adding related posts to your blog, you are now allowing a search engine spider to easily get older posts easily.
Related posts are just links to posts that were made in the past. Some may have been posted in the last week, others maybe last year. These posts have a much better change of getting updated in search engines then if they were buried in the archives.