My take is that there are two interests in self driving cars for Google Proper (i.e. Search, Maps, Youtube, News, Ads).
* Free version of Uber/Lyft based on a Google ad/maps referral as described by this patent [1]
* Free up peoples commute times so there eyeballs are free to use Google more (watch Youtube, use Google Search, read Google News). Perhaps even access free in-Waymo wifi that only allows Google domain access.
They have monopolies in these two areas and therefore neither requires 100% ownership in Waymo.
[1] https://techcrunch.com/2014/01/23/google-awarded-patent-for-...