Sometimes, your activity (or other piece of Android code) will need to speak up.
Not every interaction with Android users will be tidy and containable in activities composed of views. Errors will crop up. Background tasks may take much longer than expected. Something asynchronous may occur, such as an incoming message. In these and other cases, you may need to communicate with the user outside the bounds of the traditional user interface.
Of course, this is nothing new. Error messages in the form of dialog boxes have been around for a long time. More subtle indicators also exist, from task tray icons to bouncing dock icons to vibrating cell phones.
Android has quite a few systems for letting you alert your users ...