"Background Mode", your Pvt & Attn Posts are periodically synced up in the background (like a Messaging app). These posts are thereafter, available OFFLINE.
Use Background Mode, if your Internet connection is Intermittent or Slow. Pvt/Att Posts will automatically get synced up when connection is available. When you click on the Notifications, the latest posts will be IMMEDIATELY available, irrespective of whether internet connection is Slow or completely Off.
Coming Soon: Offline Blog/Pic/Trip Posting. In Background Mode, Blogs/Pics will get "posted" immediately, irrespective of whether you are Offline or Online or Internet Connectivity is slow. Thereafter, the pics/blogs will get uploaded in the Background as and when connection is available.