Here's why: You can't go back to “regular” iCloud syncing, and your Mac (and any mobile Apple devices running iOS 7 or earlier) can't access anything synced to iCloud Drive. This is a feature coming to Yosemite, the next version of OS X, and that hasn't been released yet. Even if you have the Yosemite public beta installed on your Mac this is still not a reliable way to sync data.
Complicating this further is the fact that some apps you use on both OS X and iOS use iCloud syncing for keeping data current on all devices, but they aren't using iCloud Drive to sync yet, because Mavericks can't access it. For instance, if like using, say, Realmac Software's Clear, you can't enable this feature or you'll lose functionality on your Mac until the as-yet-unannounced release of Yosemite.
All you have to do is is choose “Not Now” on the iCloud Drive screen, which DOES point out you will be losing sync functionality to your other devices (and helpfully lists them) if you do say yes. If you don't use iCloud Drive, everything should work just like before. So skip iCloud drive for now, and enjoy the rest of the new features in iOS 8!