Microsoft has purchased Palo Alto-based starup Double Labs, maker of the free Android Echo Notification Lockscreen app.

Microsoft Chief Experience Officer Julie Larson-Green disclosed the Double Labs acquisition -- with no dates or price paid -- to Business Insider late last week. Microsoft officials confirmed the report to VentureBeat, providing no further details.

Double Labs' free Android lockscreen app works on Android phones and small (10-inch and under) Android tablets. It shows users full notifications on their lock screens.

Many are wondering why Microsoft bought an Android lockscreen vendor, especially given Microsoft's own Garage incubator already launched an Android lockscreen. That Microsoft app, Next Lock Screen, is available in the Google Play Store.

So why buy an identical free app from a startup? It's all about productivity and mobility.

Microsoft has been buying up some of the biggest iOS and Android productivity apps over the past several months. It bought Acompli and quickly rebranded its email product as "Outlook" for iOS and Android. It bought iOS and Android calendar vendor Sunrise and to-do-list vendor 6Wunderkinder.

Microsoft already had products that provide the same features in many, if not all, of the categories where these acquisitions play. But Microsoft is opting to buy apps with already established mind share and market share with iOS and Android users in order to get its own brand and offerings in front of these non-Windows-Mobile users.

Larson-Green told Business Insider Android, because of its market share, is "a great platform for rapid experimentation." It's also, at least in theory, a great platform for making money if and when Microsoft can hook more Android users on its apps and services, starting with free and hopefully moving on to paid.