A bit delayed but finally you can get a free wildcard certificate for a domain.
So, what’s it for?
For creating encrypted connections between web server and browser.
Check out the Wikipedia article.
Can’t you just use OpenSSL?
Yes you can use any ssl library to create a self signed certificate. But you will get annoying warnings from the browser.
This is to get a certificate that is trusted by all major browsers, so you get that green lock in the url bar in the browser.