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I use it because I can easily insert Latex into it for math. I also use it to generate automated reports (e.g. update this document you made last year with new data).

I think the best use case for Markdown is when you specifically want 'not too rich text' – for example, I like the way it's used on Stack Exchange. It needs to be combined with another tool, like a web application, to take care of layout.

Markdown fits in well with Git.

Asciidoc has richer features than Markdown.