Benedict Evans wrote about what Google needs on mobile.

Over time Android has also evolved to provide reach in collecting data as well – you’re always logged in to Google on your Android phone, and it knows where you are when you do that search or open that app, and where everyone else who ever did that search was, and what they did next (this is one reason why retaining control of the Android UI, and heading off forks, matters to Google). There’s an old computer science saying that a computer should never ask a question that it should be able to work out the answer to – the sensors and other capabilities in smartphones in general and Android in particular massively expand the range of things that Google can work out. So, Android transforms Google’s reach both in collecting and surfacing data.