Some trends are awesome, and some are not so much. This one counts as the former. Android manufacturers have increasingly turned to the Play Store for distributing updates to their core Android apps. Sony's been doing this for a while now, and the Xperia Keyboard is its latest upload.
When Google uploads an app to the Play Store, such as the new camera app released yesterday, it tends to open the software up to more devices (though not always). With other manufacturers, including Sony, this isn't usually the case (but again, not always). Regardless, this allows for a speedier and more transparent means of distributing updates to users. Heck, it sure beats waiting for a firmware upgrade.