[mfn]

One benefit is that it's portable: your text will be rendered properly with the correct formatting anywhere markdown is supported (github, forum comments, blog post, etc). It essentially frees you from having to worry about layout and visual styling, which shouldn't interfere with the writing process.

[rat87]

There's no markdown standard (although I think some implementers were talking about it) beyond the original implementation, some implementations may not even support that I believe hacker news supports a subset. It is portable in the sense that you get(probably browser portable) simple html after running it through a probably open source markdown implementation as opposed to complex word formatting formats like odf or docx.