Think of users working from their phones or tablets. The point is to keep to simple. But I don't think I'd agree with the title of this piece from Slate. You have to know you'll be scaling to the desktop, too.
Forget the Desktop
"This isn’t a radical proposal. Many Web designers build their sites according to this principle—Jason Kottke and John Gruber’s blogs
were designed for desktop browsers, but because they load quickly and
aren’t cluttered by extraneous elements, they look like they were built
with mobile devices in mind."