From my experience, one thing that most spammers do is try to post comments, pingbacks or trackbacks on your extremely old posts.
If you are not monitoring the comments, pingbacks or trackbacks on your site properly, you will not even realize that these have been posted.
Many a times, you would not want anyone to post a comment or a pingback on an extremely old post, especially on personal blogs.
WordPress has no inbuilt system of closing comments on old posts. This is where the plugin steps in.
Auto-Close Comments, Pingbacks and Trackbacks will close comments, pingbacks and trackbacks on your post automatically at intervals set by you. You can also choose to disable the schedule and run it manually whenever you choose to.