This is a really interesting move. It provides some protection against anti-monopoly actions, as r6y9yd8lar5u1mugbz said, and the fascinating prospect of opening up additional funding avenues and risk-protection as kleinbl00 said. Another interesting dimension is PR – other Alphabet subsidiaries won't benefit from the inherent value attached to the Google brand nearly as much as if they were directly Google products, "Glass, from the company owned by the company who owns Google" just doesn't have the same ring to it. On the other hand if a risky Alphabet subsidiary fails as dramatically as Google+ did, that failure won't reflect on Google and damage its brand nearly as badly. This seems like a win from so many perspectives: Ease of administration, risk avoidance, monopoly defence, additional funding, tax mitigation, brand management. I'm kind of surprised they didn't do this sooner, on reflection.