What is the use case for the Google Wi-Fi Assistant? The Google Wi-Fi assistant is available on the Pixel and Nexus devices running Android 5.1+, in some countries. The description at Settings->Google->Networking is 


  Wi-Fi assistant:
  Automatically connect to Wi-Fi at select locations and other open networks determined to be high quality




Why would I want to use Wi-Fi assistant? I am not accessing anything on Google's network that requires authentication, and if an application requires secure data, it should already be using an SSL/TLS connection. 

From a privacy point of view, this seems to trade the wireless network's ability to monitor all traffic with Google's ability to monitor all traffic through the VPN.
The Google Wi-Fi Assistant is designed to provide a seamless always-connected experience using Wi-Fi. If, for example, you do not have a mobile data connection, the Wi-Fi Assistant will connect you to "high-quality" Wi-Fi whenever possible, and provide security through VPN.