A primary benefit of Android is that Google is allowed to put their search engine on Android mobile devices for close to free instead of multiple billions of dollars.
How much are customers buying $200 phones worth to advertisers compared to customers paying 3 times as much?
Google also has to pay third party Android OEMs a share of search revenue.
Google pays Apple to make Google the search engine on the iPhone for the same reason that Coca Cola and Anheiseur-Busch spend tens of millions per year on 30 second spots during the Super Bowl.