iOS 11 feature prevents an iPhone from automatically connecting to a weak Wi-Fi signal
The beauty of this new feature is that if you have "Ask to Join Networks" turned on, all of the weak Wi-Fi networks that would send you notifications asking you to connect to their signal, will no longer bother you.
You might recall that a couple of years ago Apple included "Wi-Fi Assist" with iOS 9. With this feature, if the Wi-Fi signal you are connected to starts to lose strength and the signal drops off, you are automatically connected to your carrier's cellular network. However, when the feature first launched, it was enabled by default. As a result, many iPhone users thought that they were connected to a Wi-Fi signal when in reality, they had been kicked off a weak Wi-Fi network and were using their carrier's pipeline. This caused some to ring up large overages with one teenager getting hit with a $2,000 bill. Eventually, a Class Action suit was filed. Now, Wi-Fi Assist does not come enabled out of the box.
The new feature found in iOS 11 beta 4 won't put you on a cellular network, so you don't have to worry about racking up overages. And it will keep you off weak Wi-Fi signals, so in some ways this is a win-win situation for iOS users.
source: @rjonesy via TheVerge