They don't want people thinking of Google when their cars are holding up traffic, turn out to be assholes when merging or changing lanes, or cause a fatal accident. Thus, Waymo.
In some sense, a single entity Google is an illusion so best to avoid it.
I'm also guessing it helps lure in potential leaders like John Krafcik (CEO of Waymo), because these execs can come onboard with the title of CEO rather than some VP within Google. (though, Krafcik joined Google 1 year before Waymo was founded).
TBH most of the articles I've read that mention Waymo are in relation to their lawsuit with Uber, and many of them say "Alphabet" and not "Google", and Alphabet still doesn't register as Google in my mind.
The printing press that is advertising there steals all the focus.
I'm convinced that if Waymo was a division of Google, they wouldn't be close to monetizing.
It seems Porat felt that Google would be in a better position by separating all that out so that things are more transparent to investors. If the stock price is any indication of how investors felt about that, she was right.
This doesn't mean that the Google brand couldn't still be applied to Waymo's activities, as is the case with Google Fiber, which kept the name. But it is not just Waymo that lost the Google brand in the process: Google X became just X, and Google Life Sciences became Verily.
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Google has failed sooo hard in so many things it should be trading for $10 if it mattered.
The real effect is that, when google starts the in-car ad attack, people will act surprised.
Google's search and ad network is paying (directly or indirectly) for everything. All other projects are just to keep people using their search and seeing ads on their platform.
What an odd business.
There are many Alphabet companies: Calico, CapitalG, Chronicle, DeepMind, Jigsaw, Nest Labs, Sidewalk Labs, Verily, Waymo. Is the idea that all of these were created under the same PR-driven motivation? I think it's much more reasonable that it was a legitimate re-org of a massive company, and that organizational units want their own identity and autonomy.
With existing Google shareholders getting some ownership.
Alphabet would retain controlling interesting. This is pretty common but not the only way. Sometimes parent company shareholders are given shares in the new company directly. So say every share of GOOG might be given a share of Waymo Inc for example. It is not always the case that the parent company keeps control.
It can also happen the other way where a sub is public and the parent does an IPO.
The structure of Alphabet is what gives the flexibility to do this.