Those times when you’re typing a document at work on a shonky Windows 7 machine, and longing for your Mac, where you’d have professional text-handling tools, like Marked for previewing Markdown.
Not that you can’t preview, as long as you’ve got a decent text editor such as Sublime Text (well, specifically Sublime). But things are just so much easier with Mac tools.
And I speak as one who has never had the opportunity to use the Mac professionally. I’ve used Windows machines at works since about 1993, and before that green-screen 5250 terminals.
One of these days, though.