Lets consider, You have couple of articles based on SEO, if a visitor have arrived at your site while searching the word SEO, then he/she might want to check more articles that you have written in your site.
As I think many of then prefer to search the blog instead of browsing every content. That’s what I mean to say, Implementing Google Custom Search in the Post will let them to search the related content easily and more rapidly.
Remember: the eCPM of AdSense for Search is more than AdSense for Content!