Old blog posts are underutilized traffic generators. While you are unaware, they can collect dust on your blog's oldest pages. They can reappear as a diamond in the SERPs.
Highly visible content (typically the most popular) may appear straight at the top of organic search results or even in a featured snippet. Still, less visible information may not surface until searchers scroll down to page two. You won't believe what you're about to hear.
It's important to update your blog posts regularly. Depending on the blog site and your style, you may find it easier to update your old posts than new ones. However, it's also important to remember that everyone's blog sees a different version of the world. For your old posts to be of any use, you also need to update your new posts similarly.