http://wccftech.com/android-8-0-oreo-wifi-autofill/

A stable Wi-Fi connection on phones is what we need as it eats up less battery and offers more functionality. Usually, whenever the Wi-Fi connection conks off, our cellular data (if enabled) gets eaten up without us realising.

With Android 8.0 Oreo, Google has tackled the issue of stable Wi-Fi connection. The latest version of the OS features the ability to connect to a known Wi-Fi network automatically whenever in close range even when the Wi-Fi is not enabled. It solves the problem of open Wi-Fi connection that scans every network in range and casts a negative impact on battery, on Oreo, it automatically connects to known access points and hence saves battery. So, if your device is on Oreo, you can turn off Wi-Fi when stepping outside and it will automatically connect to your Wi-Fi when you are near a known network. A nifty addition to the OS to save the battery, we say!