You are correct that Google search results for deep linking to your application are achievable using only universal links and naturally display your web content in Google search results.
The Google documentation is small, but there are two main reasons why you want to integrate the SDK for indexing Google applications:
First, as the documentation states, Google rates your “app content” higher in Google’s search results than if you hadn’t integrated the SDK. I assume that “application content” is defined here as your web content with metadata associated with the application, for example, the <link rel=alternate> specified in the documentation. I have not yet found a way to check / check the extent to which this helps - we must trust the documentation at the moment.
Secondly, your deep binding will only work with Safari Mobile. To facilitate deep linking of Google search results to your application in Chrome for iOS , you will need to implement the Google SDK, which will also add a back to Google button in your application. To find out if this is worth doing, you will need to check your analytics to find out how many of your iOS users use Chrome instead of Safari Mobile as the main browser on the iPhone.
Source for the second reason: https://www.youtube.com/watch?v=6NFRNamQGCc&feature=youtu.be&t=268
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