Fast forward today, when you make a Google search on your mobile, it displays results both from the mobile web as well as mobile apps. Just like mobile web, the result displayed is a deep link that takes the user to a specific piece of content within the app rather than launching the first screen of the app.
Now how is it relevant to your business? Say you run a niche fashion store specializing in waistcoats. Someone googles for linen waistcoats on their mobile set. Now since your niche fashion store has one of the most relevant results for the query, Google indexes your app and displays it in top search results. Google app indexing not only puts your app at the forefront among the top players, but also prompts the prospects to download your app (since every result comes with an app install prompt by default). The users click on the link and lands directly on the specific searched content within the app.