Summed up: convenience, and curiosity.
I can lay in bed and ask if to turn off the lights, and all lights in the house shut off (including the ones the kids forgot to turn off). That convenience is phenomenal.
And my kids who are too young to own their own devices can ask it questions. Whatever is on their random little minds, which very frequently leads them to a topic that they then come to us to discuss in depth. I love that it helps them foster their curiosity about the world around them and often enough leads to healthy conversations with Mom or Dad.
Usually by the second or third question they got as much out of Google Assistant as they are going to get (because they just keep asking variations of the same thing, but don't quite grasp that it is really the same question, or they have now gotten to a hyper specific question). By question 5 or 6 they typically are done with Google and come to us to discuss further.