Apple introduced a new Wi-Fi feature in iOS 9 called Wi-Fi Assist. With its help, an iOS device can quickly revert to cellular data if available to improve the performance of a shaky Wi-Fi network. As soon as iOS 9 became widely available, many users have started noticed their data usage started increasing considerably after upgrading to Apple’s latest mobile operating system, and Wi-Fi Assist was believed to be the culprit. iPhone users that have limited data traffic have been consequently advised to disable Wi-Fi Assist on their phones, which is turned on by default.
Apparently, Apple disagrees with that line of thinking. The company released a detailed help document that further explains how Wi-Fi Assist works.
Read more here:: Boy Genius Report