Alphabet broadly allows Google to continue-on as Google, in the event that Waymo or its other broader subsidiaries fail (and conversely, allows Google to be cut if that part starts to die). Alphabet is probably one of the most tactical "renames" out of the bunch.
Alphabet isn't quite the same thing, though. They kept the Google name around, and created Alphabet as a kind of holding company. This way they retained the value of their brand while also getting to have a kooky company with a fun name.
