从官方文档看，应该是为了获得其他APP的通知内容从而在自己的锁屏界面上显示。

Lock Screen Notifications

With the release of Android 5.0 (API level 21), notifications may now appear on the lock screen. Your app can use this functionality to provide media playback controls and other common actions. Users can choose via Settings whether to display notifications on the lock screen, and you can designate whether a notification from your app is visible on the lock screen.

http://developer.android.com/guide/topics/ui/notifiers/notifications.html#lockscreenNotification

获取通知可以不显示啊，偷偷上传啊

是作为系统通知层窗口来展示的，你可以去看一下WindowManager.LayoutParams的type属性。我做过一个类似的功能，是将自己的设置为TYPE_SYSTEM_ERROR这样的层级，确保自己在所有窗口的最上面。