I thought of one!
It shows up when your blog is linked to elsewhere, making a nice little thumbnail for people to click on. Not so on MN, but if you're on the NM network too (shh!), or on technorati or any other blog indexing site, then it just looks more interesting than a link with no photo.
Also sharing blog posts on Facebook always looks better with a thumbnail, IMHO.
AND if you have a WP blog, then the mobile swipe version looks a gazillion times better with photos for each post.
#thatsall