CSS is providing newer and more complex methods of laying out your pages. Given the multiple form factors a responsive site has to support, it makes sense that developers want easy ways to structure the layouts that aren’t all floats, clears and position: absolutes.
Regardless of how you want your layout to appear in a browser, you must keep in mind that a clear HTML structure is important to search engines. Sorry, while the bit about search engines is true, it’s not really what I consider important, but it is more likely to get some people to pay attention.
What I consider important are users of screen readers or any kind of assistive technology that allows a user to navigate a page by regions (typically by the Tab↹ key), as well as sighted keyboard users.