Mostly because it's fun in a nerdy way, and it was an interesting challenge to implement.

I decided to use it for a demos page on my site: http://mrcoles.com/demo/

Also, there's a bookmarklet at the top of the markdown-css demo that let's you apply the markdown styles to any page, which is like a harder-to-read version of readability or a quick way to see how well layed out the markup is on any page.