Since every network interaction with your server could wake the wireless radio interface up, on a device with limited energy resources it is crucial to minimize the number of times that your application tries to hit the network to sync data.
For applications that require regular updates and up-to-date data, like a messaging application, polling in the background by setting an alarm for every x minutes, then waking up the radio and downloading your data could drain the battery in a matter of hours.
Figure 1 - Polling data from remote server
The GCM offers us a platform to efficiently deliver notifications, with less than 4096 bytes, when there is new data to be consumed or to synchronize. This interaction model reduces the network interactions, and there is no need to constantly poll the server to discover data changes.
Beyond the ability to dispatch downstream messages from your server (using HTTP or XMPP protocol messages) to your Android application, the GCM framework...
