It's important to take advantage of WiFi in this era of capped data plans, but what if you forget to turn your WiFi on? That might happen much less often in Android O. There's a feature (not currently active) that can turn WiFi on for you when you get near a saved network.

From the limited information available, it seems this feature will turn your WiFi on automatically based on your location. Google knows where your phone is, and it has a good idea of what networks are around thanks to background scanning. It's not much of a jump to imagine the phone turning WiFi back on when there's a saved network in range. So, turn it off when you're out and about, and the phone will re-enable when prudent.

There's a toggle for this feature in the O preview WiFi settings, but it's grayed out along with the open network notification. Although, it's off whereas the network notification is stuck on. Presumably this will be accessible in a future version of Android O. We've still got three more builds before the final one.