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.