Obviously web design work should be done in HTML, or a very close approximation thereof (HAML). However, despite the link title above, Markdown isn't a web design language at all. It's for writing and editing copy, which is a very different task.
Let's say you have a bunch of untrained writers who needs to do some semantic formatting. They don't know HTML, so you can't ask them to just write HTML. If you let them run rampant with a WYSIWYG word processor (probably Word) you'll wind up with monstrosities where fonts were manually bolded and enlarged to indicate headers and such, and someone has to clean up after it.
If you have Markdown editors, though, you can turn an untrained writer loose with a cheat sheet ("write # to get a header" and such) and you're all set. As a bonus, you can put the Markdown document in version control, export to formats besides HTML, and a few other useful things.
Some major websites like reddit have also successfully made use of Markdown and its competitors to permit rich text comments from people who know nothing about HTML.
Let's say you have a bunch of untrained writers who needs to do some semantic formatting. They don't know HTML, so you can't ask them to just write HTML. If you let them run rampant with a WYSIWYG word processor (probably Word) you'll wind up with monstrosities where fonts were manually bolded and enlarged to indicate headers and such, and someone has to clean up after it.
If you have Markdown editors, though, you can turn an untrained writer loose with a cheat sheet ("write # to get a header" and such) and you're all set. As a bonus, you can put the Markdown document in version control, export to formats besides HTML, and a few other useful things.
Some major websites like reddit have also successfully made use of Markdown and its competitors to permit rich text comments from people who know nothing about HTML.