Sure, the wireless network at your home or office may be reasonably reliable and speedy, but what about when you connect your iPhone to a coffee shop or other public access point? It can get spotty and slow. Presumably, the idea behind Wi-Fi Plus Cellular is that your phone will still show a preference for using Wi-Fi where possible, but it will fall back on your 3G connection when it feels like the Wi-Fi isn’t pulling its weight. This way, you don’t have to manually toggle between Wi-Fi and 3G just to get the job done.
This feature showed up in the most recent beta build of iOS 6 and it looks like you’ll be able to cherry-pick which apps are able to fall back on 3G. This way, you’re not completely bogging yourself down. Set priority to your mail, for instance, plus your iCloud Documents, iTunes purchases, Passbook, and Reading Lists. Again, as with so many other Apple innovations, the goal here is to make sure things “just work.” Even if you are leeching off some terrible Wi-Fi.
Via Apple Insider