When writing in text mode so that you can see what you are generating. Wouldn't really need it the other direction though.

Isn't the point of Markdown that you don't need to 'see what you are generating', you can just read it? I write Markdown every day, but I do it in a plaintext editor (with syntax highlighting). I have a keyboard shortcut to view a preview in my browser, but I don't see a great need to be viewing that preview all the time.

I think the point of Markdown is to signal that you're someone who uses things like Markdown.

Markdown is ubiquitous even in boring corporate technical writing processes nowadays. Nothing hip about it.