This is all about featuring content in WP.
Say, you got 1000's of posts that you accumulated over the years and each has their own original ( and true ) post dates.
And now you want to feature them again.
Say in your category taxonomy, you have a branch called Featured and underneath are the sub items as follows;
Featured Main Headline Must See Worth A look Not to Miss etc.
And in your theme, you have a zone to display these various featured posts. As an example, you have a "must see" zone that features the last 10 posts that you did and you happened to categorize it under "must see".
The question is what do we mean here by "the last"? The last according to the post_date, right?
What if I were to feature an article that's from the past? How do I do that? Just selecting the "must see" is not going to help because as I add more and more items to the "must see" category, how do I display them in the order I featured them. One way to tackle this is to change the post_date I guess? But would not that mess up the archival information. I have a hard time justifying changing the original post date just for the sake of featuring things in proper order.