c markdown html,Markdown or HTML

I have a requirement for users to create, modify and delete their own articles. I plan on using the WMD editor that SO uses to create the articles.

From what I can gather SO stores the markdown and the HTML. Why does it do this - what is the benefit?

I can't decide whether to store the markdown, HTML or both. If I store both which one do I retrieve and convert to display to the user.

UPDATE:

Ok, I think from the answers so far, i should be storing both the markdown and HTML. That seems cool. I have also been reading a blog post from Jeff regarding XSS exploits. Because the WMD editor allows you to input any HTML this could cause me some headaches.

The blog post in question is here. I am guessing that I will have to follow the same approach as SO - and sanitize the input on the server side.

Is the sanitize code that SO uses available as Open Source or will I have to start this from scratch?

Any help would be much appreciated.

Thanks