Brett Terpstra gives six reasons to use Markdown:
It’s clean: Markdown translates quickly to perfectly-formed HTML. No missing closing tags, no improperly nested tags, no blocks left without containers.
I started using Markdown for blogging right around the time I launched this blog. Now I don’t know what I would do without it. It’s so easy and fast, and so much more readable than HTML. I love it. Brett does a great job outlining it for anyone that hasn’t used it, but has thought about it.