At the bottom of each post is the word "Permalink". A permalink is a unique web link that: a) is dedicated solely to that post and b) remains fixed to that post indefinitely.
Bloggers link to each others posts and sites. As new posts are added to blogs older ones are archived. Because they move off of the page and to other areas of the blog a dedicated link ensures that if someone links to the post … that there is no "page cannot be displayed" experience.