So far Google using App Indexing has already been helping people to engage Android app after they’ve installed it. Google also indicates that they have well over 30 billion links within apps indexed.
Google has now made next step, as of this week people searching on Google can also discover an app even if they haven’t installed it yet.
If you’re an app developer and have implemented App Indexing, then there is a big chance that if content from an app is relevant to a search done on Google on Android devices, people may start to see app install buttons for this app in search results. These buttons will take visitor to the Google Play store where they can install the app, then continue straight on to the right content within it.
Source:http://googlewebmastercentral.blogspot.co.uk/2015/04/drive-app-installs-through-app-indexing.html