A related posts WordPress plugin is one of the most important plugin for a pro blog. It helps users to locate more posts of your blog which are probably old or in some way are not receiving the attention that they deserve.
Here are the advantages of related posts plugin:
1) Makes content more visible to your visitors: There are several posts in a blog which in spite of your best efforts do not receive many page views as anticipated. In Google Analytics dashboard, go to Behavior –> Site content –> All pages. Drilldown to the last result page and you will find some of the posts with one page view. These are the some of the posts which didn’t rank very well in search engines and probably were not shared by many people in the social networks. Now, when you use a related posts plugin, you can add a related posts widget after your post content as you see in this blog. This will increase the chances of the old posts being visited by users.
2) Increase the time spent on your blog by visitors: From the day I have installed the related posts plugin, I have seen an increase in Average visits duration of visitors in my blog.
3) Increase in pages per visit and decrease in bounce rate: Related posts plugin increases the pages per visit metric of your blog as some of your visitors will be inclined towards reading posts which are related to the original post. Thus, bounce rate for you blog will be reduced.
Disadvantage of related posts plugin: Related posts plugin has only one disadvantage, that it is a resource intensive plugin. Plugin performs queries in the background for matching related posts for a particular post. If you are using a shared hosting, you might would like to test it for a few days on how it goes.
My number one choice for a related posts plugin is Yet another related posts plugin (YARPP). It has many options and settings which are easy to configure.