My own guess: it allows upcoming management/leadership to exercise more independence in the various divisions of "Google". Further, People like titles, and it's hard to share them.
A very good point. Didn't look like Sundar Pichai was going to oust Larry Page as CEO anytime soon. This gives everyone the ability to continue their upward mobility (and responsibility), with increased independence from aligned (but separate) businesses within Goog--er, Alphabet.
IOW, Nest reports to Alphabet, and so does Google. Both are aligned, but separate.
