It's a given today, each blog post should have at least an image. Wordpress is more aware of this and have options to easily add images/videos to posts. But we often need a specific image for a post that can used by wp themes in separate locations from text content. Something like slideshows on home page or stylish display of posts in home page (as in screenshot above) needs this.
Wordpress added an option called 'Featured Image' as early as it's version 2.9. But it's not intuitive & not all themes avail this either.