It’s not a WP specific thing – 3 column layouts with semantic source ordering (as it’s called) is a bugger-hard thing to do. Fortunately, there are a few places I’ve seen that show exactly how to do it – all the hacks needed for different browsers, all the caveats related to it’s use, etc.
The benefit is, natch, backwards compatibility – or “graceful degradation”. Old browsers should see the content before the sidebar menus ??