I think the way to think about it is Google is doing what it does best: search. They use whatever content they can scrape from the screen using the view hierarchy once the user signals intent that they want to search. It's a faster way to do search using context (in this case the screen) to provide the "keywords". The app indexing part is how they are able to show related apps with deep links. They use app indexing of other apps to find relevant results for the query.

App indexing makes sense when you have more stable content inside of an app like a traditional public website. Think Yelp and their 'screen' per place inside of the app. It doesn't make sense to publish the contents of a chat conversation to Google's app index -- that would be a huge privacy leak! It would be the equivalent of GoogleBot indexing my Gmail inbox.