The first reason is more or less common sense.  Someone searches for “how to do X and Y” and I have a page indexed that is “how to do X.”  Now somewhere on my site I may have another blog post that answers their question more completely.  Perhaps that page is new and hasn’t been indexed yet.  Or perhaps the SE user didn’t know exactly what they were looking for and after finding my page with their botched or incomplete search terms they find what they were actually looking for in a related post.

The second and most important reason to use the related post is for SEO purposes.  The main purpose is “interlinking.”  If you don’t know, interlinking simply is defined as the internal link structure within your own site.  Now suppose your get a high authority link to a certain page on your site about oranges.  Google now thinks that this page is important and relavant information.  Perhaps your post about oranges mentions fruit a good bit.  Now your related posts display other fruit related posts, like posts on grapefruit and apples.  With the related posts plugin you are actually pushing your own link juice so that now google sees that your other pages are relavant and important also.

Also another thing to consider is your indexing saturation.  This can be figured out by dividing the number of pages you have indexed in google by the number or pages that are on your site.  If you don’t know how to figure out the number of pages has index then read:

Know about your site and your competition – Google Operators

The related post plugin helps drastically with index saturation by pushing google to some of your older posts that happen to be relevant.  This is particularly useful if you have a large number of posts(1000+).  Now their are several related post plugins out their, and sadly I haven’t tried many of them.  The one I use is here:

WordPress Recent Post Plugin

So our objective is to get all of the comments that we have made noticed by google bot so we can pick up our sweet links.  If you simply post a comment, then there is a reasonable chance that search engines will never pick it up.  The solution is to gently guide the search engines to check out those links.  I’m going to give you a quick easy way to do this.  Please, be creative and try different ways to make this work for you.

1. First thing to do is to make a list of all of the pages that you made a comment on.  If you make a comment, and it goes through, save the link in a notepad document.

2. Now sign up for an account at Wordpress.com or Blogger.com and start a blog.  Now write a post that includes links to all of the comment pages that you saved.

3. Now sign up for an account at digg.com.  Now submit your wordpress or blogger post that has the links to digg.

That is all there is to it.  If all goes well your digg page will be indexed quickly.  It will spit google bot out at your free blog, and your blog will spit google out to all of the links that have your comments on them.  Now I’m not saying that this is a fool proof method or 100% successful.  This method can however be applied in a number of different ways boosting its effectivness.

Note:  Don’t use your own blog to post links to the comments you made.  You’ll be turning your 1 way links into recipricol links and screwing up the link weight.