Let’s imagine that you need personal blog. You have some slow free hosting, it has no PHP and no ability to use any server-side at all. Or even worse - you only have account on Dropbox, and you need your blog there.
Or maybe you prefer Markdown-syntax to writing posts in HTML, more by token Markdown format is supported by all popular editors and many web-writing platforms. It would be convenient to store posts in separate files to make it easy to copy one post to Tumblr ect. Also it gives an opportunity to store fles in repository or organize subversioning of articles.