From what I've read, this was strictly a decision to appease people internally at Google to avoid name dilution and to make it possible to put more focus on each of their products and services.
Google had become such a MASSIVE conglomerate that the name wasn't really resonating the way they wanted it to. Search engines, video services, operating systems, self-driving cars, wearable computers, even weather balloons providing wi-fi internet access, all are bearing the Google name, but there's also things Google owns which you wouldn't even begin to think of as Google properties, including health-related companies and an organization based on promoting climate change awareness.
So, speaking strictly from a business standpoint, in order to keep the Google name resonating with technology and internet services, they turned the company into Alphabet, are taking some aspects of Google and shifting them to be parts of Alphabet instead, and Google will continue to be a massive aspect of the company, just with less of the unnecessary bulk people don't typically associate Google as having.
Basically, the large conglomerate known as Google is now known as Alphabet which has a "smaller" piece of itself known as Google.
Yeah, it's a bit weird to wrap one's head around but it actually makes perfect sense and I get why Google would do this. The trouble is that the average person is NOT going to get this and instead is just going to latch onto it as Google stupidly changing their name to something ridiculous like Alphabet and that everything known as Google will suddenly be known as Alphabet... which yeah, that's really stupid, which is why that's NOT what's happening. :P
--- Kris Asick (Gemini)
--- Pixelmusement Website: www.pixelships.com
--- Ancient DOS Games Webshow: www.pixelships.com/adg